A Gravatar stands for globally recognized avatar.
Gravatar are simply small (static or animated) image of aroung 80*80 pixel generally found at weblogs.
These small graphic are used for identification purpose, so visitors can make you out easily just by looking at the gravatar.
These graphics are placed beside your name in weblogs. Not all weblogs provides facility for uploading of these gravatars.
These are similar to Avatar which is part of todays forums.
Near future would be of talking avatars which can boost your online presence and would increase your traffic. Now-a-days these are becoming more popular.