If you didn’t know about microformats

A microformat (sometimes abbreviated as uF is a web-based approach to semantic markup that seeks to re-use existing XHTML and HTML tags to convey metadata and other attributes. This approach allows information intended for end-users (such as contact information, geographic coordinates, calendar events, and the like) to also be automatically processed by software.

-wikipedia

In a bit more simpler meaning, if you have a software which can understand a web page written with microformat, it would be possible for the software to automatically recognize the informations like, contact information, geometric information, bookmarks etc.

If it is still not very clear, what will extra benefit will you get from it, just try operator firefox addon once with Layers theme. The theme uses hatom, hcard, XFN, rel-home, xFolk microformats.

The article by otto helped a lot for this update.