What are the Exo in Bungie's Destiny?

The Exo are character class / player species that players can choose to play as Guardians in Bungie's Destiny Game.

The Exo have been described as the Terminators of the Destiny universe, brutal killing machines that would make Sky Net happy. Joe Staten said the Exo were, "sinister, powerful, and tireless."Or or so it would seem, this above concept art shows an Exo clearly in a contemplative mode, perhaps after a hard fought battle.

Members of the Exo species have been described as being physically intimidating, tending to prefer armour and using technology over magic or stealth.