A gravatar, or globally recognized avatar, is quite simply an image that follows you from site to site appearing beside your name when you do things. Avatars help identify your posts as your own, on blogs and web forums, so why not on any site?
Signing up for a gravatar.com account is FREE, and all that's required is your email address. Once you've signed up you can upload your avatar image and soon after you'll start seeing it on gravatar enabled sites!
This saves you a lot of time uploading your image every time you want to action on the Internet. It will help you build that online identity.
If you have your own website, you can use the Gravatar plug in. You don't need an account for this. Since all gravatars are rated with an MPAA style rating, you can restrict your site to show only gravatars whose content you are comfortable with.