According to one of the definitions I find online, an avatar is...
The manifestation of a Hindu deity (especially Vishnu) in human or superhuman or animal form; "the Buddha is considered an avatar of the god Vishnu".
So in that sense, we are all avatars. We - as human beings - are Existence, God, Buddha Mind, Brahman, etc. manifesting, expressing and exploring itself in human form.
The difference between those that get the label and those who don't is that in some, this is a living realization and in others it is not. Apart from that, there seems to be no significant difference.