"Avatars are icons or representations of users in a shared virtual reality. Within online virtual communities and Internet forums, avatars are pictures that users display alongside their contributions to represent themselves. Avatars have also become popular in instant messaging and are also referred to as buddy icons."
http://www.pinkmagazine.com/exclusives/gen_y_terms_aug.sept2006.html
The avatar function allows you to either upload an image into the forum, OR if you already have a gravatar, choose this option and allow the forum software to find your gravatar info via your email address.
