The Death Eaters were an organisation of dark wizards formed under the command of Lord Voldemort. They believe in Blood-purity and are in favour of the use of dark magic, including Unforgivable curses. They wear hoods to conceal their identities and many do not know the names of the others within their organisation as a means of protection.
The Death Eaters largely disbanded following Voldemort's loss of his powers upon attacking Harry Potter, and many were arrested and sent to Azkaban. They were re-formed upon Voldemort's rebirth in the graveyard in Little Hangleton and later assumed control of the Ministry of Magic, prior to Voldemort's second defeat.
Some referred to Voldemort as the "Chief Death Eater."