It’s a pretty cute idea, and much less threatening than the full-on identity management concepts like Passport and Typekey. Basically you just associate an image with your email address and that’s it! Any blog [or web page] can request that image from a centralized server, and wha-a-a-lah! Portable thumbnail! See the comments section for example of how it appears…
I first encountered gravatars on Binary Bonsai, and as you can see from this page, it’s quite a nice way to identify commenters.