Sub-genre of Metal where the musicians are extremely talented and play as fast and technical as humanely possible, yet still wont hardly sell any records or get the recognition they deserve.
Song structure is typically like classical music: It's almost non existent. Melody changes ever few seconds, drums change on a dime, and singing (or growling, pig squeals, screaming, etc.) often pops in and out.
Changing melodies typically make death metal less memorable to casual fans, which is why you don't really see it on MTV (Well, that and the death growl vocals creep people out).
There is also Melodic death metal, where the growl is replaced with a scream, the melody doesn't change as much, and keyboards are typically added, and deathcore, which is a fusion of hardcore and death metal. These genres are sometimes criticized by death metalheads for changing true death metal.
Death metal: Necrophagist, Morbid Angel, Nile, Cannibal Corpse, Deicide, Dying Fetus, Behemoth, Death, Prostitute Disfigurement, Alchemist, Possessed, Atheist, God Dethroned
Melodic Death Metal: In Flames, Dark Tranquillity, Detonation
Deathcore: Job for a Cowboy (early), Despised Icon, I Killed the Prom Queen
If you listen to death metal and all you hear is noise and random banging of drums, you don't deserve to listen (or criticize) it.