A Gravatar is a Globally Recognized Avatar. You upload it and create your profile just once, and then when you participate in any Gravatar-enabled site, your Gravatar image will automatically follow you there.
An "avatar" is an image that represents you online—a little picture that appears next to your name when you interact with websites (like the one shown to the left).
Gravatar is a free service run and supported by Automattic.