A heavier form of rock and roll music, that is definately not for everybody. However, if you like music that is violent, heavy, and generally badass, I recommend going to your nearest record store and buying a Priest or Maiden album. If you like rock music, but wish it was heavier, you won't regret it, trust me.
There are many different types of metal. I'll list a few examples of each genre:
Heavy Metal: Black Sabbath, Judas Priest, Iron Maiden, Tank, Accept, Motorhead, Armored Saint, Saxon.
Thrash Metal: SSSSLLLLAAAAYYYYEEEERRRR!!!!(the yelling is necessary, trust me), Kreator, Sodom, Testament, Overkill, Exodus.
Death Metal: Death, Morbid Angel, Possessed, Bolt Thrower, Monstrosity, Deicide, Vader.
Power Metal: Sabaton, Hammerfall, Gamma Ray, Blind Guardian, Iced Earth, Helloween.
Black Metal: (a.k.a. the gods rock and roll of which you need to lay down your soul to) Bathory (old), Celtic Frost, Burzum, Dark Throne, Emperor, Mayhem.
Viking Metal: Bathory (new), Ensiferum, Tyr, Korpiklaani, Enslaved.
Doom Metal: Candlemass, Solitude Aeturnus, My Dying Bride (not an emo band, surprisingly), Electric Wizard.
And that's just a few. Also, don't think that metal is all a bunch of shouting about Satan. It's not. Trust me, I listen to it, and it's not.