If you look at the comment section of this blog or any number of blogs on the internet you will notice an area that is available for a user image / icon / avatar. The empty space looks like this:

The little gray box with the white silloquette is where the Gravatar goes. What, you ask? [G]lobally [r]ecognized avatar or Gravatar. From Automattic:

A gravatar, or globally recognized avatar, is quite simply an avatar image that follows you from weblog to weblog appearing beside your name when you comment on gravatar enabled sites. Avatars help identify your posts on web forums, so why not on weblogs?

Automattic is also behind the WordPress publishing platform. To sign up for a Gravatar you will need to go to http://en.gravatar.com/site/signup/ and enter your email address. Before you sign up find a nice photo of yourself because you will need it in order to set up your own Gravatar.

Gravatar works by recognizing your email address when you submit comments to blogs. When you set up your account you can register several email addresses. Once you have it all set up you will see comments like this:

Pretty slick, huh. Give it a shot @ http://en.gravatar.com/site/signup/