Gravatar (globally recognizable avatar) is a globally accessible avatar storage service, used, in particular, by StackOverflow.
Gravatar is a service, that stores unique user identifier pictures - "avatars". Stored avatar can be accessed through URL containing MD5 hash of user's e-mail. If supplied MD5 was not rcognized, service generates an unique image pattern for it.
This tag is appropriate for marking questions concerning Gravatar usage and structure (e.g. embedding Gravatar images into guestbook engine, etc)