Roughly speaking, metal can be pretty cleanly divided–or at least as cleanly divided as any artistic endeavor can be–into five styles: traditional, fast, slow, alternative and symphonic. There are genres and subgenres within each of those styles, but the lines are a lot less clear.
Traditional metal consists of the first few waves of heavy metal, the power metal that really took off in the 80s and 90s, and bands who are playing in either of those styles. The major bands in this style are Judas Priest, Iron Maiden, Helloween and Stratovarius.
Fast metal is like traditional metal, but played faster. Power metal grew into thrash metal, which grew into death metal. Some hardcore punk bands changed up their sound and became either old school punk metal, late 90s crossover, or modern metalcore. Others went even more extreme and started playing grindcore. Early industrial metal falls in here as well. Notable bands are Metallica, Megadeth, Anthrax, Slayer, Pantera, Napalm Death, Ministry, Morbid Angel, Carcass, Deicide, early Corrosion of Conformity, At the Gates, early In Flames, Vision of Disorder, Refused and As I Lay Dying (who are the biggest metalcore band that I’m familiar with).
Slow metal started out as a reaction to the movement towards faster metal and away from Black Sabbath’s sounds. There’s a significant amount of Sabbath worship, but also some more adventurous bands who either play for the sweet leaf crowd or play very slowly to make music that not everyone will enjoy. Note that except for the extreme doom, “slow” is a relative notion. Some goth metal and grunge falls into this category as well. Major bands are Candlemass, St. Vitus, later Corrosion of Conformity, Crowbar, Monster Magnet, Kyuss, Orange Goblin, Electric Wizard, Thergothon, Skepticism, Cathedral, Theatre of Tragedy, the Melvins and Alice in Chains.
Alternative metal is the heavy music of a younger generation, who weren’t quite old enough to participate in the first few waves of heavy metal. In addition to traditional heavy metal, bands playing in this style have been heavily influenced by rap, the so called funk metal of the late 80s and early 90s, and electronic music. Modern industrial metal and some metalcore influenced acts fall in here as well. Major acts include Helmet, Korn, Disturbed, Rage Against the Machine, Killswitch Engage, later In Flames, Marilyn Manson, later Within Temptation and System of a Down.
Symphonic metal is metal that’s influenced by traditional European music. This includes not only classical/symphonic music, but folk music as well. Black metal and viking metal fall in this category, as well as folk metal and what some have called dark opera. Look for work by Bathory, Emperor, Darkthrone, Enslaved, Einherjer, Tyr, Korpiklaani and Lacrimosa.
As always, there are some bands that don’t fall neatly into any category (most notably Opeth, who play death metal influenced prog rock). I’ve also left out very closely related types of music, such as hard rock (which is where most of the hair metal acts fall), hardcore and crust punk, and more traditional industrial. I’ve also left out bands who are very strongly metal influenced, but don’t really play music that can be described as metal (most notably Arcturus, but also stuff like Ulver or Mother Earth-era Within Temptation).
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