After reading this thread, it depends on the time that you grew up.
Led Zeppelin, Black Sabbath, Cream, and Deep Purple are the roots of metal.
Then you have:
New Wave of British Heavy Metal with bands like Iron Maiden, Motorhead, Venom, and Def Leppard (yes they can be considered metal)
Then the influencies of NWOBHM:
Metallica, Megadeth, Motley Crue, and Guns 'n Roses
From there different Genres started to arise
Thrash Metal - Slayer, Pantera, Sepultura, Anthrax
Doom Metal - Type O Negative, Sons of Otis
Death Metal - Children of Bodom, Morbid Angel, In Flames
Power Metal - Dragonforce, Iced Earth, Meshuggah, Dream Theatre
All of the above influenced
Metal Core - Killswitch Engage, Soilwork, As I Lay Dying, Shadows Fall
Emo - I don't listen to this music but Bullet for my Valentine is a good example
Pop Metal - Avenged Sevenfold
I am a bit drunk and just looking though itunes, so please correct me if I am wrong on any of my rant!!
METAL RULES!!!!!! LOVE LIVE DOUBLE BASS, SQUEELS, CLIFF BURTON and RANDY RHODES!