Post type refers to the various structured data that is maintained in the WordPress posts table. Native (or built-in) registered post types are post, page, attachment, revision, and nav-menu-item. Custom post types are also supported in WordPress and can be defined with register_post_type(). Custom post types allow users to easily create and manage such things as portfolios, projects, video libraries, podcasts, quotes, chats, and whatever a user or developer can imagine.

Reference http://codex.wordpress.org/Glossary