Gravatar stands for Globally Recognised avatar.
It indicates part of a growing trend to have a one size fits all item for a user, wherever they go on the Net. We saw this first with user names eg Open ID.
The calls are growing for social networking sites eg facebook, Myspace, Linkedin etc to collaborate so that an individual can carry not just their ID with them, but also profile info, photos, videos etc etc wherever they go, and these are shared across the different sites. Otherwise you can spend your life filling in the same information across a multitude of sites!
Gravatars look fun, as well as being useful in establishing brand, corporate or individual identity, and it si easy to change your gravatar so that the change is reflected across many of teh sites that you and your company interact with.