Post Formats are meta information that can be used by themes to customize presentation of a post. The basic idea is to provide a specific method for specifying the display “format” of a post. This replaces the need to use categories to accomplish the same thing.
Mark Jaquith announced on the WordPress Development blog that themes will be able to add support for post formats like this:
add_theme_support( 'post-formats', array( 'aside', 'gallery' ) );
This is how it will show up on the post listing screen:
There’s your little peak at some of the cool things that are coming in WordPress 3.1. Make sure you’re subscribed and we’ll keep you up to date!