The core is based on 11 database tables:

1. $wpdb->posts - basic posts, pages and other post types info;
2. $wpdb->postmeta - extra posts, pages and other post types info;
3. $wpdb->comments - basic comments info;
4. $wpdb->commentmeta - extra comments info;
5. $wpdb->links - links info;
6. $wpdb->options - options info;
7. $wpdb->users - basic user info;
8. $wpdb->usermeta - extra user info;
9. $wpdb->terms - categories and tags
10. $wpdb->term_relationships - relationship between posts and categories or tags;
11. $wpdb->term_taxonomy - relationship for categories or tags;

All table names is the concatination of WordPress table prefix and table name (for example: "wp_posts"). WordPress table prefix ($wpdb->prefix) is "wp_" by default. You can change it in "wp-config.php" if you want to intall more than one WordPress site on the same database.

Working with database in WordPress.