The Death Eaters are the henchmen of Lord Voldemort. The group primarily consists of wizards and witches who were radical pure blood supremacists, who practice the Dark Arts with reckless abandon and without regard to or fear of wizarding law.
Lord Voldemort used this select group of wizards and witches during both the First and Second Wizarding Wars, employing them as his elite force. They often wore black hoods and masks with snake-like eye slits to cover their faces. Only the members of the inner circle of the Death Eaters had Dark Marks burned into their left forearms. When Voldemort touched one of them, each of the Death Eaters felt it, signalling for them to Apparate to their leader's side.
Severus Snape (a double-agent trusted by both Albus Dumbledore and Voldemort)
Bellatrix Lestrange (Voldemort's most fanatic supporter, and his most ruthless and sadistic crony)
Draco Malfoy (Harry Potter's school rival; he becomes a Death Eater during his sixth year, with orders to assassinate Dumbledore, but becomes uneasy about it as he realizes the dark deeds his father was forced to do in the past)
Lucius Malfoy (one of Voldemort's inner circle, but loses favor after failing several duties; ultimately defects)