...First, it's a cartoon people, not a science textbook!!!! Yeesh, why aren't you complaining about how airbenders live in the high mountians where there's little air!? Or about the 'Spirit World'?!?

Okay, and according to what the show seems to be going on - mythology from about 264623409 different cultures - Air is an element. In either China or Japan the elements were/are Earth, Fire, Water, Metal, and Air. In some Pagan mythos, there's an element called 'Spirit'. The show has little basis in fact, and in fact most of the things in the show come from mythology: 'Agni' is one of the forms of Shiva; the whole reincarnation thing; karma; Gods and Goddesses of various types ((La-Ocean, Agni-Fire, etc.)).

Here's something: "Avatar draws on four of the five classical Indian elements of Hindu and Buddhist traditions for the four bending arts: Fire (agni or tejas), Water (ap or jala), Earth (prithvi or bhumi), and Wind or Air (vayu or pavan). The fifth, aether (akasha or akash) is symbolized by Aang because as the Avatar, he is the intermediary between the physical world and the Spirit World. The four elements can also be found in Western traditions from at least the 5th century BC. Some names in the series, such as "Agni Kai" and King Bumi the Earthbender, borrow directly from these elements. The four elements, Fire, Air, Water, and Earth represents the four states of matter: plasma, gas, liquid, and solid."~Quote from Wikipedia on Avatar: The Last Airbender.