diff --git a/API.md b/API.md
index 609977f..5e30d55 100644
--- a/API.md
+++ b/API.md
@@ -40,7 +40,9 @@
 3. [Resources](#resources)
 
    - [User](#user)
+   - [Tag category](#tag-category)
    - [Tag](#tag)
+   - [Snapshot](#snapshot)
 
 4. [Search](#search)
 
@@ -146,9 +148,9 @@ data.
     {
         "tagCategory": <tag-category>,
         "snapshots": [
-            {"data": <tag-category-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-category-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-category-snapshot>, "time": <snapshot-time>}
+            <snapshot>,
+            <snapshot>,
+            <snapshot>
         ]
     }
     ```
@@ -188,9 +190,9 @@ data.
     {
         "tagCategory": <tag-category>,
         "snapshots": [
-            {"data": <tag-category-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-category-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-category-snapshot>, "time": <snapshot-time>}
+            <snapshot>,
+            <snapshot>,
+            <snapshot>
         ]
     }
     ```
@@ -223,9 +225,9 @@ data.
     {
         "tagCategory": <tag-category>,
         "snapshots": [
-            {"data": <tag-category-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-category-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-category-snapshot>, "time": <snapshot-time>}
+            <snapshot>,
+            <snapshot>,
+            <snapshot>
         ]
     }
     ```
@@ -275,17 +277,15 @@ data.
 
     ```json5
     {
-        "query": "haruhi",
+        "query":    <query>, // same as in input
+        "page":     <page>,  // same as in input
+        "pageSize": <page-size>,
+        "total":    <total-count>,
         "tags": [
-            <tag>,
-            <tag>,
             <tag>,
             <tag>,
             <tag>
-        ],
-        "page": 1,
-        "pageSize": 5,
-        "total": 7
+        ]
     }
     ```
     ...where `<tag>` is a [tag resource](#tag) and `query` contains standard
@@ -373,9 +373,9 @@ data.
     {
         "tag": <tag>,
         "snapshots": [
-            {"data": <tag-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-snapshot>, "time": <snapshot-time>}
+            <snapshot>,
+            <snapshot>,
+            <snapshot>
         ]
     }
     ```
@@ -427,9 +427,9 @@ data.
     {
         "tag": <tag>,
         "snapshots": [
-            {"data": <tag-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-snapshot>, "time": <snapshot-time>}
+            <snapshot>,
+            <snapshot>,
+            <snapshot>
         ]
     }
     ```
@@ -469,9 +469,9 @@ data.
     {
         "tag": <tag>,
         "snapshots": [
-            {"data": <tag-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-snapshot>, "time": <snapshot-time>}
+            <snapshot>,
+            <snapshot>,
+            <snapshot>
         ]
     }
     ```
@@ -519,8 +519,8 @@ data.
 
     ```json5
     {
-        "remove": "source-tag",
-        "merge-to": "target-tag"
+        "remove":   <source-tag-name>,
+        "merge-to": <target-tag-name>
     }
     ```
 
@@ -530,9 +530,9 @@ data.
     {
         "tag": <tag>,
         "snapshots": [
-            {"data": <tag-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-snapshot>, "time": <snapshot-time>},
-            {"data": <tag-snapshot>, "time": <snapshot-time>}
+            <snapshot>,
+            <snapshot>,
+            <snapshot>
         ]
     }
     ```
@@ -565,11 +565,11 @@ data.
         "siblings": [
             {
                 "tag": <tag>,
-                "occurrences": 2
+                "occurrences": <occurrence-count>
             },
             {
                 "tag": <tag>,
-                "occurrences": 1
+                "occurrences": <occurrence-count>
             }
         ]
     }
@@ -597,17 +597,17 @@ data.
 
     ```json5
     {
-        "query": "rr-",
+        "query":    <query>, // same as in input
+        "page":     <page>,  // same as in input
+        "pageSize": <page-size>,
+        "total":    <total-count>,
         "users": [
             <user>,
             <user>,
             <user>,
             <user>,
             <user>
-        ],
-        "page": 1,
-        "pageSize": 5,
-        "total": 7
+        ]
     }
     ```
     ...where `<user>` is a [user resource](#user) and `query` contains standard
@@ -865,62 +865,171 @@ data.
 # Resources
 
 ## User
+**Description**
+
+A single user.
+
+**Structure**
 
 ```json5
 {
-    "name":          "rr-",
-    "email":         "rr-@sakuya.pl",    // available only if the request is authenticated by the same user
-    "rank":          "admin",            // controlled by server's configuration
-    "rankName":      "Administrator",    // controlled by server's configuration
-    "lastLoginTime": "2016-04-08T20:20:16.570517",
-    "creationTime":  "2016-03-28T13:37:01.755461",
-    "avatarStyle":   "gravatar",         // "gravatar" or "manual"
-    "avatarUrl":     "http://gravatar.com/(...)"
+    "name":          <name>,
+    "email":         <email>,
+    "rank":          <rank>,
+    "rankName":      <rank-name>,
+    "lastLoginTime": <last-login-time>,
+    "creationTime":  <creation-time>,
+    "avatarStyle":   <avatar-style>,
+    "avatarUrl":     <avatar-url>
 }
 ```
 
+**Field meaning**
+- `<name>`: the user name.
+- `<email>`: the user email. It is available only if the request is
+  authenticated by the same user.
+- `<rank>`: the user rank, which effectively affects their privileges. The
+  available ranks are stored in the server configuration.
+- `<rank-name>`: the text representation of user's rank. Like `<rank>`, the
+  possible values depend on the server configuration.
+- `<last-login-time>`: the last login time, formatted as per RFC 3339.
+- `<creation-time>`: the user registration time, formatted as per RFC 3339.
+- `<avatarStyle>`: how to render the user avatar.
+
+    Possible values:
+
+    - `"gravatar"`: the user uses Gravatar.
+    - `"manual"`: the user has uploaded a picture manually.
+
+- `<avatarUrl>`: the URL to the avatar.
+
 ## Tag category
+**Description**
+
+A single tag category. The primary purpose of tag categories is to distinguish
+certain tag types (such as characters, media type etc.), which improves user
+experience.
+
+**Structure**
 
 ```json5
 {
-    "name":  "character",
-    "color": "#FF0000" // used to colorize certain tag types in the web client
+    "name":  <name>,
+    "color": <color>
 }
 ```
 
-## Tag category snapshot
+**Field meaning**
 
-```json5
-{
-    "name":  "character",
-    "color": "#FF0000"
-}
-```
+- `<name>`: the category name.
+- `<color>`: the category color.
 
 ## Tag
+**Description**
+
+A single tag. Tags are used to let users search for posts.
+
+**Structure**
 
 ```json5
 {
-    "names":        ["tag1", "tag2", "tag3"],
-    "category":     "plain",
-    "implications": ["implied-tag1", "implied-tag2", "implied-tag3"],
-    "suggestions":  ["suggested-tag1", "suggested-tag2", "suggested-tag3"],
-    "creationTime": "2016-03-28T13:37:01.755461",
-    "lastEditTime": "2016-04-08T20:20:16.570517"
+    "names":        <names>,
+    "category":     <category>,
+    "implications": <implications>,
+    "suggestions":  <suggestions>,
+    "creationTime": <creation-time>,
+    "lastEditTime": <last-edit-time>
 }
 ```
 
-## Tag snapshot
+**Field meaning**
+
+- `<names>`: a list of tag names (aliases). Tagging a post with any name will
+  automatically assign the first name from this list.
+- `<category>`: the name of the category the given tag belongs to.
+- `<implications>`: a list of implied tag names. Implied tags are automatically
+  appended by the web client on usage.
+- `<suggestions>`: a list of suggested tag names. Suggested tags are shown to
+  the user by the web client on usage.
+- `<creation-time>`: time the tag was created, formatted as per RFC 3339.
+- `<creation-time>`: time the tag was edited, formatted as per RFC 3339.
+
+## Snapshot
+**Description**
+
+A snapshot is a version of a database resource.
+
+**Structure**
 
 ```json5
 {
-    "names":        ["tag1", "tag2", "tag3"],
-    "category":     "plain",
-    "implications": ["imp1", "imp2", "imp3"],
-    "suggestions":  ["sug1", "sug2", "sug3"]
+    "operation":    <operation>,
+    "type":         <resource-type>
+    "id":           <resource-id>,
+    "user":         <user-name>,
+    "data":         <data>,
+    "earlier-data": <earlier-data>,
+    "time":         <time>
 }
 ```
 
+**Field meaning**
+
+- `<operation>`: what happened to the resource.
+
+    The value can be either of values below:
+
+    - `"created"` - the resource has been created
+    - `"modified"` - the resource has been modified
+    - `"deleted"` - the resource has been deleted
+
+- `<resource-type>` and `<resource-id>`: the resource that was changed.
+
+    The values are correlated as per table below:
+
+    | `<resource-type>` | `<resource-id>`                 |
+    | ----------------- | ------------------------------- |
+    | `"tag"`           | first tag name at given time    |
+    | `"tag_category"`  | tag category name at given time |
+    | `"post"`          | post ID                         |
+
+- `<user-name>`: name of the user who has made the change.
+
+- `<data>`: the snapshot data.
+
+    The value can be either of structures below:
+
+    - Tag category snapshot data (`<resource-type> = "tag"`)
+
+        *Example*
+
+        ```json5
+        {
+            "name":  "character",
+            "color": "#FF0000"
+        }
+        ```
+
+    - Tag snapshot data (`<resource-type> = "tag"`)
+
+        *Example*
+
+        ```json5
+        {
+            "names":        ["tag1", "tag2", "tag3"],
+            "category":     "plain",
+            "implications": ["imp1", "imp2", "imp3"],
+            "suggestions":  ["sug1", "sug2", "sug3"]
+        }
+        ```
+
+- `<earlier-data>`: `<data>` field from the last snapshot of the same resource.
+  This allows the client to create visual diffs for any given snapshot without
+  the need to know any other snapshots for a given resource.
+
+- `<time>`: when the snapshot was created (i.e. when the resource was changed),
+  formatted as per RFC 3339.
+
 
 # Search
 
diff --git a/server/szurubooru/api/tag_api.py b/server/szurubooru/api/tag_api.py
index 6783cd2..748cc0b 100644
--- a/server/szurubooru/api/tag_api.py
+++ b/server/szurubooru/api/tag_api.py
@@ -18,7 +18,7 @@ def _serialize_tag(tag):
 def _serialize_tag_with_details(tag):
     return {
         'tag': _serialize_tag(tag),
-        'snapshots': snapshots.get_data(tag),
+        'snapshots': snapshots.get_serialized_history(tag),
     }
 
 class TagListApi(BaseApi):
@@ -89,6 +89,7 @@ class TagDetailApi(BaseApi):
             tags.update_implications(tag, ctx.get_param_as_list('implications'))
 
         tag.last_edit_time = datetime.datetime.now()
+        ctx.session.flush()
         snapshots.modify(tag, ctx.user)
         ctx.session.commit()
         tags.export_to_json()
@@ -126,8 +127,8 @@ class TagMergeApi(BaseApi):
             raise tags.InvalidTagRelationError(
                 'Cannot merge tag with itself.')
         auth.verify_privilege(ctx.user, 'tags:merge')
-        tags.merge_tags(source_tag, target_tag)
         snapshots.delete(source_tag, ctx.user)
+        tags.merge_tags(source_tag, target_tag)
         ctx.session.commit()
         tags.export_to_json()
         return _serialize_tag_with_details(target_tag)
diff --git a/server/szurubooru/api/tag_category_api.py b/server/szurubooru/api/tag_category_api.py
index 627ee53..19420a3 100644
--- a/server/szurubooru/api/tag_category_api.py
+++ b/server/szurubooru/api/tag_category_api.py
@@ -10,7 +10,7 @@ def _serialize_category(category):
 def _serialize_category_with_details(category):
     return {
         'tagCategory': _serialize_category(category),
-        'snapshots': snapshots.get_data(category),
+        'snapshots': snapshots.get_serialized_history(category),
     }
 
 class TagCategoryListApi(BaseApi):
@@ -56,6 +56,7 @@ class TagCategoryDetailApi(BaseApi):
             auth.verify_privilege(ctx.user, 'tag_categories:edit:color')
             tag_categories.update_color(
                 category, ctx.get_param_as_string('color'))
+        ctx.session.flush()
         snapshots.modify(category, ctx.user)
         ctx.session.commit()
         tags.export_to_json()
diff --git a/server/szurubooru/db/snapshot.py b/server/szurubooru/db/snapshot.py
index 7772982..cd9af7a 100644
--- a/server/szurubooru/db/snapshot.py
+++ b/server/szurubooru/db/snapshot.py
@@ -5,7 +5,7 @@ from szurubooru.db.base import Base
 class Snapshot(Base):
     __tablename__ = 'snapshot'
 
-    OPERATION_CREATED = 'added'
+    OPERATION_CREATED = 'created'
     OPERATION_MODIFIED = 'modified'
     OPERATION_DELETED = 'deleted'
 
@@ -13,6 +13,7 @@ class Snapshot(Base):
     creation_time = Column('creation_time', DateTime, nullable=False)
     resource_type = Column('resource_type', String(32), nullable=False)
     resource_id = Column('resource_id', Integer, nullable=False)
+    resource_repr = Column('resource_repr', String(64), nullable=False)
     operation = Column('operation', String(16), nullable=False)
     user_id = Column('user_id', Integer, ForeignKey('user.id'))
     data = Column('data', PickleType)
diff --git a/server/szurubooru/func/snapshots.py b/server/szurubooru/func/snapshots.py
index 5d366ab..100f823 100644
--- a/server/szurubooru/func/snapshots.py
+++ b/server/szurubooru/func/snapshots.py
@@ -19,44 +19,69 @@ def get_tag_category_snapshot(category):
 
 # pylint: disable=invalid-name
 serializers = {
-    'tag': get_tag_snapshot,
-    'tag_category': get_tag_category_snapshot,
+    'tag': (
+        get_tag_snapshot,
+        lambda tag: tag.first_name),
+    'tag_category': (
+        get_tag_category_snapshot,
+        lambda category: category.name),
 }
 
 def get_resource_info(entity):
-    table_name = entity.__table__.name
+    resource_type = entity.__table__.name
+    assert resource_type in serializers
+
     primary_key = inspect(entity).identity
-    assert table_name in serializers
     assert primary_key is not None
     assert len(primary_key) == 1
-    primary_key = primary_key[0]
-    return (table_name, primary_key)
+
+    resource_repr = serializers[resource_type][1](entity)
+    assert resource_repr
+
+    resource_id = primary_key[0]
+    assert resource_id
+
+    return (resource_type, resource_id, resource_repr)
 
 def get_snapshots(entity):
-    table_name, primary_key = get_resource_info(entity)
+    resource_type, resource_id, _ = get_resource_info(entity)
     return db.session \
         .query(db.Snapshot) \
-        .filter(db.Snapshot.resource_type == table_name) \
-        .filter(db.Snapshot.resource_id == primary_key) \
+        .filter(db.Snapshot.resource_type == resource_type) \
+        .filter(db.Snapshot.resource_id == resource_id) \
         .order_by(db.Snapshot.creation_time.desc()) \
         .all()
 
-def get_data(entity):
+def serialize_snapshot(snapshot, earlier_snapshot):
+    return {
+        'operation': snapshot.operation,
+        'type': snapshot.resource_type,
+        'id': snapshot.resource_repr,
+        'user': snapshot.user.name,
+        'data': snapshot.data,
+        'earlier-data': earlier_snapshot.data if earlier_snapshot else None,
+        'time': snapshot.creation_time,
+    }
+
+def get_serialized_history(entity):
     ret = []
-    for snapshot in get_snapshots(entity):
-        ret.append({'data': snapshot.data, 'time': snapshot.creation_time})
+    earlier_snapshot = None
+    for snapshot in reversed(get_snapshots(entity)):
+        ret.insert(0, serialize_snapshot(snapshot, earlier_snapshot))
+        earlier_snapshot = snapshot
     return ret
 
 def save(operation, entity, auth_user):
-    table_name, primary_key = get_resource_info(entity)
+    resource_type, resource_id, resource_repr = get_resource_info(entity)
     now = datetime.datetime.now()
 
     snapshot = db.Snapshot()
     snapshot.creation_time = now
     snapshot.operation = operation
-    snapshot.resource_type = table_name
-    snapshot.resource_id = primary_key
-    snapshot.data = serializers[table_name](entity)
+    snapshot.resource_type = resource_type
+    snapshot.resource_id = resource_id
+    snapshot.resource_repr = resource_repr
+    snapshot.data = serializers[resource_type][0](entity)
     snapshot.user = auth_user
 
     earlier_snapshots = get_snapshots(entity)
diff --git a/server/szurubooru/migrations/versions/46cd5229839b_add_snapshot_resource_repr.py b/server/szurubooru/migrations/versions/46cd5229839b_add_snapshot_resource_repr.py
new file mode 100644
index 0000000..819f947
--- /dev/null
+++ b/server/szurubooru/migrations/versions/46cd5229839b_add_snapshot_resource_repr.py
@@ -0,0 +1,22 @@
+'''
+Add snapshot resource_repr column
+
+Revision ID: 46cd5229839b
+Created at: 2016-04-21 19:00:48.087069
+'''
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = '46cd5229839b'
+down_revision = '565e01e3cf6d'
+branch_labels = None
+depends_on = None
+
+def upgrade():
+    op.add_column(
+        'snapshot',
+        sa.Column('resource_repr', sa.String(length=64), nullable=False))
+
+def downgrade():
+    op.drop_column('snapshot', 'resource_repr')
diff --git a/server/szurubooru/tests/func/test_snapshots.py b/server/szurubooru/tests/func/test_snapshots.py
index 3bd44af..4e421b5 100644
--- a/server/szurubooru/tests/func/test_snapshots.py
+++ b/server/szurubooru/tests/func/test_snapshots.py
@@ -173,33 +173,49 @@ def test_merging_deletion_all_the_way_deletes_all_snapshots(
         snapshots.delete(tag, user)
     assert db.session.query(db.Snapshot).count() == 0
 
-def test_get_data(fake_datetime, tag_factory, user_factory):
+def test_get_serialized_history(fake_datetime, tag_factory, user_factory):
     tag = tag_factory(names=['dummy'])
-    user = user_factory()
+    user = user_factory(name='the-user')
     db.session.add_all([tag, user])
     db.session.flush()
     with fake_datetime('2016-04-19 13:00:00'):
         snapshots.create(tag, user)
     tag.names = [db.TagName('changed')]
+    db.session.flush()
     with fake_datetime('2016-04-19 13:10:01'):
         snapshots.modify(tag, user)
-    assert snapshots.get_data(tag) == [
+    assert snapshots.get_serialized_history(tag) == [
         {
+            'operation': 'modified',
             'time': datetime.datetime(2016, 4, 19, 13, 10, 1),
+            'type': 'tag',
+            'id': 'changed',
+            'user': 'the-user',
             'data': {
                 'names': ['changed'],
                 'category': 'dummy',
                 'suggestions': [],
                 'implications': [],
             },
-        },
-        {
-            'time': datetime.datetime(2016, 4, 19, 13, 0, 0),
-            'data': {
+            'earlier-data': {
                 'names': ['dummy'],
                 'category': 'dummy',
                 'suggestions': [],
                 'implications': [],
             },
         },
+        {
+            'operation': 'created',
+            'time': datetime.datetime(2016, 4, 19, 13, 0, 0),
+            'type': 'tag',
+            'id': 'dummy',
+            'user': 'the-user',
+            'data': {
+                'names': ['dummy'],
+                'category': 'dummy',
+                'suggestions': [],
+                'implications': [],
+            },
+            'earlier-data': None,
+        },
     ]