An "avatar" is an image that represents you online—a little picture that appears next to your name when you interact with websites.
A Gravatar is a Globally Recognized Avatar. You upload an image and create your public profile just once, and then when you participate in any Gravatar-enabled site, your Gravatar image and public profile will automatically follow you there.
Gravatar is a free service for site owners, developers, and users. It is automatically included in every WordPress.com account and is run and supported by Automattic.