Large number of free and premium WordPress plugins simply change values in already existing WP database. Learn about WP #database now.
Did you know that everything inside WordPress is actually a post? So, if you create Post, Page, Attachment, Revision, Custom CSS, Changesets… everythig is stored inside “wp_posts” database, only with different post_type value.
Also, all your users are stored inside “wp_users” database, and all addtional data about these users (profile fields for example), are stored inside “wp_usermeta” table.