WordPress can hold and display many different types of content. Internally, these are all stored in the same place, in the wp_posts table. These are differentiated by a column called post_type.

WordPress 3.0 gives you the capability to add your own custom post types and to use them in different ways.
For more details check WordPress Codex

All About WordPress Custom Post Types and Taxonomies

Following articles describe WordPress custom post types in details.