The aeons are listed in the order you find them.

Valefor	Bird-like thing - Controls Air.
Ifrit	Bear with horns - Controls Fire.
Ixion	Horse - Controls Lightning and Thunder.
Shiva	'Ice Queen' - Controls Ice (duh).
Bahamut	Giant reptile with wings and two feet - Controlls Air, and is mor powerful than Valefor.
Yojimbo	Japanese Bodyguard (Yojimbo means bodyguard in Japanese) - Has cool sword.
Anima	Hard to describe - Controlls the existence between life and death, if you get what I'm saying.
The Magus Sisters	One fat, one skinny, and one a bee - Are really awesome.

The Fayth were people who sacrificed their lives for the rest of the world. Once summoned, the Fayth can transform in to the aeons. The Eternal Aeon exists and cannot be summoned until a sacrifice is made. He is different. Normally (read top part) but he is different. You can only summon him once and then the summoner himelf/herself dies.

I am Anima's lost brother.