Since the introduction of Custom Post Type in WordPress 3.0 and Custom Taxonomies in WordPress 2.9, WordPress has been widely used as Content Management System by developer.
At it most simple definition all question that concern the use of Custom Post Type and / or Custom Taxonomy can have this tag, whether it is as a plugin or as a theme. Other than those two feature, the use of WordPress as CMS also may incorporate some sort modification on the wp-admin pages (metaboxes, admin menus and or dashboard-widgets).
Some reference on Custom Post Type : (credit to @ MikeSchinkel)
- Custom post types in WordPress
- Rock-Solid WordPress 3.0 Themes using Custom Post Types
- Everything You Need to Know About WordPress Custom Post Types
- Custom Post Types in WordPress 3.0
- The WordPress Codex on Custom Post Types
- Adding Custom Field GUI to Custom Post Types in WordPress 3.0
- Explore the Power of Custom Post Types to Maximize WordPress as a CMS
- Smarter Custom Post Types