Some people are confused about the Post Formats feature that will be made available to themes in WordPress 3.1, especially how it differs from Custom Post Types.
Custom Post Types
These were poorly named. Think: Custom Content Types. That is, non-post content. Examples: employees, products, attachments, menu items, pages, pets. If you want it to show up in your site’s main RSS feed, then it’s probably not a custom post type.
A Post Format is a formatting designation made to a post. For example, a post could be a short “aside,” or a Kottke.org-style link post, or a video post, or a photo gallery post. The data you input might be slightly different — video post should contain a video, an aside should probably not be very long, a link post should have a link. And the way that the post is displayed on the site might be very different — an aside will typically be displayed without a title, a link post may have the title point to the link. A video post may be wider, or have social sharing buttons auto-appended. But they’re all still posts. They still show up in your feed, and you still find them in the Posts section of the WordPress backend.
The important thing to note about Post Formats is that they are going to be a standardized convention. So any theme that supports Post Formats and follows the standard will display your posts in a way that makes sense. Before, themes had to set up category-based conventions, and these conventions weren’t shared by other themes. This is a better way of handling that, and it should make it even easier to switch between themes than before!
Themers should turn to the Post Formats page in the Codex for info on implementation. Note that we’re not yet in beta, so expect this page to change a bit.