Garrett Dimon has written an excellent introduction to / primer for microformats. He provides simple straightforward examples of hCard, nofollow, and XOXO.
He discusses several reasons for using microformats including: Standards, CSS Convenience, Plug-and-Play JavaScript, and Machine-Readability. I like this explanation of some of the subtler essences and nuances of the microformats principle of humans first, machines second (emphasis mine):
When writing markup against deadlines and priorities, it’s easy to forget that somebody else will eventually have to maintain it. Conveniently, some of the central ideas behind microformats revolve around the fact that they are designed for humans first and created with simplicity in mind. This means you’ll have markup that is easy to understand and maintain for everyone, including:
- The engineer integrating your code next week
- You updating your code next month
- The new guy taking over your job when you get promoted next year
Tantek