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
Table where posts, static pages, custom post types, attachments, etc. are written.
Completes $wpdb->posts table. Stores additional data of posts they are also called metadata.
Table with the data on registered users.
Additional information about users, such as Name, Nickname, permissions, etc.
Meta-data for $wpdb->users table.
A table with records of comments.
Meta fields for the $wpdb->comments table.
Taxonomies — read more
A table containing basic information about each element of the taxonomy.
A table containing additional fields for the $wpdb->terms table.
A table with information about taxonomies and their descriptions.
A table linking taxonomies to content (posts, entries, etc.).
Other tables
Table with link records.
Table of options (settings).

Multisite tables

All sites in the network.
Contains the current version of each site's database. The data is updated when the database for each site in the network is updated.
Contains the data of site administrators that are created when the sites are created.
Contains users who were registered via basic WordPress registration from the page: ``Administration > Super Admin > Settings''.
Contains, the addresses of the main sites.
Site data: various options, including the site administrator.
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.
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...