Death metal is a subgenre of heavy metal. It usually employs varying tempos, heavily distorted guitars, "blast-beat" drumming, and deep growling vocals. Hasn't been taken seriously since around 1993.
Death metal is an extreme form of heavy metal, and is essentially an off-shoot of the '80's "thrash metal" trend made popular by Metallica and Slayer. Taking their cue instead from second-teir thrash bands like Venom, Hellhammer, and Kreator, death metal pioneers Death, Morbid Angel, and Possessed set out to create a brand of heavy metal guaranteed to make you and three of your buddies the scariest dudes in your junior high school.