ooh, I know what an 'avatar' is. It's a term from a religion, I believe Hindu-ism. An 'avatar' in the Hindu religion, is a *representative* of a god (one of the Hindu gods). It is sent by the god to represent it, in some sort of function, either to carry out the god's will, or to deliver a message from the god to a particular audience. There are various reasons why a Hindu god might create an avatar, either the god is too busy with other things to attend to certain tasks, or the god is so powerful that the presence of the actual god would destroy people, which the god did not want to do.
Anyway, the 'avatar' or representative of a deity could take different forms. It could be a veritable twin of the deity, with the same appearance and powers, or some other sort of creature, created (or even assigned) to carry out a specific task, or deliver a specific message.
So, anyway, one of the above posters is correct, the meaning of this word has changed, it now refers to not merely the representative of a Hindu deity, but also to the representative of a person, on various websites. Which can be (depending on the website) just a picture or phrase the person has chosen to represent themselves, or an image of the person, being controlled by the person. Human beings, as we cannot physically enter ourselves into computers or the internets, need something, an avatar, to represent them.
I learned this from a book called 'Timestorm', which was about some strange cosmic phenomena that caused different periods of history to start getting jumbled together on the planet earth. At one point, the main characters met up with a man (or maybe it was a creature, I forget, it's been a long time since I read the book) who told them that he was the 'avatar' of some scientist from a future time, and had been sent to help them fight the timestorm. The scientist could not come himself, as he was busy doing other things to fight the timestorm, back in his own future time, so he had created the avatar, which in this case was an actual living person or creature, to represent him and carry out this particular task for him.