A "gravatar" is a "globally recognized avatar". I can't remember why I signed up at gravatar.com about six months ago. Last October, the company that runs Wordpress, the software that runs this blog, bought gravatar.com, so it's not surprising that there is a Wordpress plugin to support gravatars. When I noticed that the new version of my comment preview plugin supports gravatars, I thought I'd give them a try here on my blog. I might not keep them, but it's a fun novelty for now. As implied, your gravatar should show up on other websites too, especially as Metcalfe's Law takes effect and more sites adopt them.
So go sign up and set your avatar for my blog comments!