Angels are the Helpers and Soldiers of Notch. They are dead, but clean humans and they were without sins. Angels were believed to be first discovered before the known excistance of Notch. There are very few who exist, and those who do exist are only half-angel.
Half angels were a Human-Angel hybrid. Two examples of a Half-angel would be Ben Sapphire and Terrance Bloodworth. They help out with real-world problems, as in "Kickin' Herobrine's Ass" as Ben says. They are more human than angel, so they can be a little dirty. But other than that, they can be pretty strict on people's sins.
There purpose is unknown, however it is believed that they are to defend the lands from the sickness of the Nether.
They are in heavy conflict with Night Demons, which come in the form of: