The Gravatar is a profile picture that is determined by the email provided. For example, in the comment section of the homepage, you may have seen different people's profile pictures updated with their comment. In many cases, for example with WordPress, this icon comes via Gravatar.
The way the service works is that you create a Gravatar account and add the email address you use. The service will ask you to add a profile picture for this email address. The next time you use a service where Gravatar is enabled and you use the email address you previously configured, the profile picture you set up will appear on that service.
So Gravatar is one profile picture for many different services, easily.