Tables (structure) of the WordPress Database

Posts — read more here

$wpdb->posts

Table where posts, static pages, custom post types, attachments, etc. are written.

$wpdb->postmeta

Completes $wpdb->posts table. Stores additional data of posts they are also called metadata.

Users

$wpdb->users

Table with the data on registered users.

$wpdb->usermeta

Additional information about users, such as Name, Nickname, permissions, etc.
Meta-data for $wpdb->users table.

Comments

$wpdb->comments

A table with records of comments.

$wpdb->commentmeta

Meta fields for the $wpdb->comments table.

Taxonomies — read more

$wpdb->terms

A table containing basic information about each element of the taxonomy.

$wpdb->termmeta

A table containing additional fields for the $wpdb->terms table.

$wpdb->term_taxonomy

A table with information about taxonomies and their descriptions.

$wpdb->term_relationships

A table linking taxonomies to content (posts, entries, etc.).

Other tables

$wpdb->links

Table with link records.

$wpdb->options

Table of options (settings).

Multisite tables

$wpdb->blogs

All sites in the network.

$wpdb->blog_versions

Contains the current version of each site's database. The data is updated when the database for each site in the network is updated.

$wpdb->registration_log

Contains the data of site administrators that are created when the sites are created.

$wpdb->signups

Contains users who were registered via basic WordPress registration from the page: ``Administration > Super Admin > Settings''.

$wpdb->site

Contains, the addresses of the main sites.

$wpdb->sitemeta

Site data: various options, including the site administrator.

$wpdb->users

A list of users of all sites on the network. This is a common table of users for the entire network. This is the usual WP users table, but in the multisite version two more fields are added: spam and delete.

$wpdb->usermeta

Contains metadata of users. User settings for different network sites.

The base tables of each network site

Network site tables: wp_posts, wp_options etc. The same tables are created for each network site, but with a different prefix, e.g: wp_options, wp_1_options, wp_2_options...