Wordpress has many different types of content, most common type of content is post type. WordPress has inbuilt five post types. All the post types are stored internally in wp_posts table, but has unique post_type column.
DEFAULT POST TYPES
1) Post:Its post type is post,these are normally displayed in blogs.
2) Pages: Its post type is page,these post type displayed various kind of content.
3) Attachment:Its post type is attachment, these post type contain information about file uploaded.
4) Revisions: Its post type is revision, these post type are very useful as these hold a draft post,its kind of backup of our work, in case we want to rivert we can easily do so by revisions.
5) Navigation: Its post type is nav_menu_item, and its contain information about menu system in wordpress
CUSTOM POST TYPE
These are not inbuilt post type,but one can create it manually using register_post_type(), it is inbuilt function of wordpress which define various attributes of posts like label,features,availability and other specifics.