It seems like we answer one of these every couple of weeks.
Black Metal - dark atmosphere, ususally has blastbeats, usually has raspy vocals, guitars are rarely downtuned, keyboards are sometimes used to add to the majestic/beautiful sound (bands: Emperor, Mayhem, Burzum, Aborym)
Death Metal - growled vocals, guitars downtuned, sometimes has blastbeats (bands: Death, Morbid Angel, At the Gates, Cryptopsy)
* there are many, many styles of Death Metal
Power Metal - clean vocals, lots of solos (bands: Blind Guardian, Rhapsody)
Heavy Doom - slow, downtuned guitars, lumbering, many styles of vocals (bands: Cathedral, Candlemass, Khanate)
Melodic Doom - depressing, melodic vocals, more acoustic intruments, sometimes beautiful in a sad way (bands: Katatonia, My Dying Bride, Anathema)
Stoner - laid back, more "free" flowing song structures, many types of vocals (bands: Electric Wizard, Kyuss, Queens of the Stone Age)
Industrial - lots of electronics (bands: Ministry, Red Harvest)