Ares is the god of war, and so he is a rather irrational god. He enters into people and causes rage, so that they want to go fight. Ares is sometimes said to be Aphrodite's boyfriend (though it's not clear what it means, that love and war would be attached to each other). Ares doesn't appear in stories much.
To find out more about Ares, check out these books from Amazon.com or from your library:
D'aulaire's Book of Greek Myths, by Edgar and Ingri D'Aulaire.
Greek Religion, by Walter Burkert (reprinted 1987). By a leading expert, for adults. He has sections on each of the Greek gods, and discusses their deeper meanings, and their function in Greek society.