I've been going through the list of tables on my Drupal site in prep to convert an existing single-site to a multi-site with a shared database. However, I haven't found any reference that illuminates the general purpose of the tables. I've tried to put together at least a preliminary list here. I share it in hopes that someone else might find it useful, but it is not normative or complete.
- access
- User. This is the table used by the "Access Rules" section for banning usernames, IPs, etc.
- accesslog
- Statistics. This stores the accesslog if Drupal is collecting statistics.
- aggregator_category
- Aggregator. Store the categories feeds may belong to.
- aggregator_category_feed
- Aggregator. Links feeds to categories.
- aggregator_category_item
- Aggregator. Links feed items to categories.
- aggregator_feed
- Aggregator. Store the feeds that the Aggregator module pulls from.
- aggregator_item
- Aggregator. Store the items that the Aggregator module has pulled for the various configured feeds.
- authmap
- User. This stores a map to the authentication module used to authentication each user in the database.
- blocks
- Block. This stores information about blocks provided by every module installed, including custom blocks.
- blocks_role
- Block. Stores the roles permitted to view blocks in the system.
- boxes
- Block. Administrator created blocks.
- cache
- System. The general cache of data, used by many modules.
- cache_filter
- System. I have no idea what this table is used for.
- cache_menu
- System. I have no idea what this table is used for.
- cache_page
- System. I have no idea what this table is used for.
- cache_views
- Views. Used to cache information related to views.
- client
- Drupal. Used to record Drupal client sites. I believe this is the list of sites that others have logged in from if you use the Drupal module.
- client_system
- Drupal. I'm not exactly certain what this table is for.
- comments
- Comment. Stores all comments made on your Drupal site.
- contact
- Contact. Stores the emails sent via a contact form.
- files
- Upload. Stores information about files uploaded via the file upload module (and used by some other modules too).
- file_revisions
- Upload. Associated file revisions with node revisions allowing different node revisions to have different versions of a file associated with them.
- filters
- Filter. Stores information about the body content filters used in your install.
- filter_formats
- Filter. Associates filter formats and settings with a body content filter.
- flood
- Watchdog. Used to detect floods from requests coming from a set.
- forum
- Forum. Used to link forum posts with the forum topic.
- history
- Node. Used to track which nodes are read/unread.
- locales_meta
- Locale. Something to do with language translation.
- locales_source
- Locale. Something to do with language translation.
- locales_target
- Locale. Something to do with language translation.
- menu
- Menu. Storage of menu module customizations.
- node
- Node. The main table for storing general node information, including the title, node number, dates, workflow state, but it does not store most of the actual content.
- node_access
- Node. Storage of per-node access permissions.
- node_comment_statistics
- Comment. Notes certain statistics related to the number of comments and how recently comments have been made on a node.
- node_counter
- Statistics. Records the view count for each node.
- node_revisions
- Node. Stores information about node revisions, including the main content of the node.
- node_type
- Node. Stores information about the custom node types.
- permission
- User. Stores the permissions that have been assigned to each role.
- poll
- Poll. Extra information associated with poll nodes.
- poll_choices
- Poll. Associates the available choices with a poll node.
- poll_votes
- Poll. Votes by visitors on a poll node.
- profile_fields
- Profile. Definitions of available profile fields.
- profile_values
- Profile. Profile field values associated with a particular user.
- role
- User. Assigns role IDs and names to all the roles in the system.
- search_dataset
- Search. Something to do with search.
- search_index
- Search. The search index.
- search_total
- Search. The number of times a given word appears on the site.
- sequences
- System. The current counter for each of the sequence IDs.
- sessions
- User. User session tracking data stored in the database.
- system
- System. Information about installed modules.
- term_access
- Taxonomy Access Control. Access control per category.
- term_access_defaults
- Taxonomy Access Control. Access control per vocabulary.
- term_data
- Taxonomy. Definition of taxonomy terms.
- term_hierarchy
- Taxonomy. List of parent terms for each taxonomy term.
- term_node
- Taxonomy. Table linking taxonomy terms to nodes.
- term_relation
- Taxonomy. Relationships between taxonomy terms.
- term_synonym
- Taxonomy. Alternative names for a taxonomy term.
- tinymce_role
- TinyMCE WYSIWYG Editor. Roles assigned to use TinyMCE profiles.
- tinymce_settings
- TinyMCE WYSIWYG Editor. Definition of TinyMCE profiles.
- url_alias
- Path. Path aliases recorded to nodes.
- users
- User. User records.
- users_roles
- User. Link table between users and roles.
- variable
- System. Administrative settings and other site-wide variables.
- view_argument
- Views. Definition of arguments to a view.
- view_exposed_filter
- Views. Definition of exposed filters in a view.
- view_filter
- Views. Definition of filters in a view.
- view_sort
- Views. Definition of sorting in a view.
- view_tablefield
- Views. Something to do with the views plugin.
- view_view
- Views. Basic information about a view.
- vocabulary
- Taxonomy. Definition of taxonomy vocabularies.
- vocabulary_node_types
- Taxonomy. Associates vocabularies with node types.
- watchdog
- Watchdog. Records watchdog log entries.

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