Ares was the Greek god of war, and was the son of Zeus and Hera.
He was brutal, bloodthirsty, and a coward. None of the gods liked him, except for Aphrodite, by whom he had three children: Deimos, Phobus, and Harmonia. (Aphrodites husband, Hephaestus, wasn't very happy about this). He is also the father of Kyknos, but I'm not sure how that came about. And to make things even more confusing, he and (a different) Harmonia gave birth to the Amazons (yes, the entire race).
Ares is often accompanied into war by Deimos (Fear), Phobus (Panic), his sister Athena, and Hades. Ares is symbolized by a vulture and a dog.
While his sister Athena was also a god(ess) of war, she was less bloodthirsty, and she encouraged war only when no peaceful solution could be found.
In Roman mythology, Ares is (re)incarnated as Mars, who represented the spirit of heroism and courage.