A Gravatar is a globally recognized avatar. A gravatar appears alongside your name on all gravatar enabled sites (like this one). So if you have a gravatar and you leave a comment here, your gravatar will appear besides your post. Gravatars are like your identity on the web and you can get one for free from http://www.gravatar.com
A gravatar is linked to your email ID. Whenever you comment on a gravatar enabled site, the site create a cryptic hash of your email ID (primarily so that the email ID remains unknown) and requests your gravatar image URL from http://www.gravatar.com. This saves you from registering at every site and uploading your picture or avatar one at a time.
A second use of gravatar is to have a global identity on the web which doesn’t change from site to site. No matter what your user ID for a particular site, a gravatar shall remain the same on every gravatar enabled site. And therefore any visitor shall be able to recognise you irrespective of your user ID. Since the gravatar is pulled from http://www.gravatar.com and does not reside on the individual site, when ever you change your gravatar, the changes reflect across all sites which show your gravatar.