They are a race of what resembles the undead. With a concentration in a base known as Hellmouth on the Moon this race is a definite enemy of Earth. There are five different races that we currently know about.
Little is known about the Thrall other than they prefer melee attacks with their large class. They have an armor that seems to be made apart of them. No visible eyes so they probably move about with other senses.
The Troopers appear to do the bulk of the combat for the Hive. They wield energy based rifles and possibly others. Unlike the Thralls the Troopers have helmets with built in optics.
Not much is known about this enemy but they can be seen here slinging a massive sword. They also appear to have much more armor than any of the other Hive.
Magic based units that appear to be in sort of leader role based on their apparel. They have long swaying cloaks that envelope their bodies as they fly about the battlefield. Their is a regal air in the depictions Bungie has released of them.
This one here is located in the depths of Hellmouth, a giant raging Hive unit that swings its massive limbs to crush those before it.