What’s a gravatar? It’s one of those icons that appears in association with people’s names on a blog. You’ll see them next to their comments, or in the log in box. Take a look here, for example. This commenter has his own gravatar.
On the other hand, here the commenter does not. The little silhouette image next to the commenter’s name is a default gravatar.
A gravatar can be any image at all, though traditionally you might want to just use a face shot of yourself.
Here’s mine. How do you get one? Go to the gravatar web site and sign up. Set up an account using the email address for the account you use to sign on here, upload an image, and that’s it. The best thing is, once you’ve got one set up it appears automatically wherever a site (like this one) supports them.