Content Types (Entities) in WordPress

There are five basic types of content (data types, entities) in WordPress out of the box:

All of this data is stored in the database, so I suggest you look at the structure of the WordPress database tables:

Wordpress database schema - the structure of all tables
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...