What is an avatar?
An avatar refers to a picture that represents an online user. Avatars are used all over the Internet, in places such as forums, social networks, online cummunities and multiplayer games.
Avatars allow people to represent themselves online in whatever way they want, personalize their contributions to the forum, show their interests, beliefs and personality.
They also serve rather practical purpose - it's easier for other users to immediately recognize and remember who the poster is by the distinct image, than just the username alone.
Acording to wikipedia, the use of the term avatar for the on-screen representation of the user was first used in 1985 by Richard Garriott for the computer game Ultima IV: Quest of the Avatar.