Is characterized by heavily distorted guitars, giving them a deep, rough sound, which is where the "heavy" part came from; powerful drums, and thick bass. Usually includes very complex guitar work and amazing solos.
Black Sabbath is often considered the original heavy metal band. Bands like Metallica and Iron Maiden are considered the perfectors of the genre, and pioneers of the overall 80s metal scene.
Over the years, it has evolved into several subgenres. The most popular one with metalheads is thrash metal, but there are several others such as power metal, doom metal, black metal, death metal, etc.
The most recent subgenre that's spawned in today's modern scene is nu metal. This subgenre has basically the same characteristics of traditional metal, except for the fact that guitar solos are very rare, if not included at all. This has caused many metalheads to look down upon it as "shit", "poser metal", or "mallcore" (many people also link it to "emo", as sometimes they may have similar lyrics).
Contrary to popular belief, heavy metal is NOT Satanic. Only black metal (and sometimes death metal) bands portray that kind of imagery. However, it's not because they actually worship Satan, but because they're against Christianity as a whole. Some other metal bands of other genres may occasionally use Satanic imagery in their songs, but once again, it's not because they worship Satan. It's just to add deeper meaning and emotion to their songs.
Also contrary to popular belief, it is NOT all mindless incomprehensible screaming. There is another subgenre of metal called "metalcore". These bands usually scream all the lyrics in their songs, and at the moment, metalcore is the most popular subgenre of metal. However, screaming metal bands only account for less than 25% of the entire genre. Metal bands like Iron Maiden, Stratovarius, Symphony X, and Kamelot incorporate very melodic vocals in their music, hardly ever screaming, if at all. If they do scream, it's only for a moment, to add emotion to a section of the song.
*Thrash Metal - Metallica, Slayer, Anthrax, Megadeth, Testament, Exodus
*Power Metal - Blind Guardian, Manowar, Stratovarius, Helloween, Edguy, Rhapsody of Fire
*Progressive Metal - Dream Theater, Symphony X, King's X, Opeth
*Grindcore - Napalm Death, Anal Cunt, Carcass
*Death Metal - Cannibal Corpse, Behemoth, Necrophagist, Children of Bodom
*Black Metal - Venom, Emperor, Bathory, Dimmu Borgir, Mayhem, Burzum
*Nu Metal - Korn Slipknot, Deftones, Disturbed, System of a Down
*Metalcore - Unearth, Shadows Fall, Killswitch Engage, Trivium (early albums), Hatebreed
In my opinion, all heavy metal is good. I know there are many "true metalheads" that will say all nu metal sucks, or all metalcore sucks, and that only old metal is good, but then they're not appreciating the metal genre as a whole. I say, why take your anger out on subgenres of your own favorite music, when you could take it out on those that are obscuring it further, like rap and pop?
If it's metal, it's metal. There shouldn't be any arguments about "true and false metal". It's just music. A guitarist doesn't have to be fucking Kirk Hammet or Kerry King to be talented. A song doesn't have to include a blazing fast solo like in Iron Maiden's Hallowed Be Thy Name to be good. Bands don't have to sound like the big names from the 80s to be great.
And speaking of big names.. Metallica, Slayer, Iron Maiden, etc. are NOT the only good metal bands around. Granted, most of today's mainstream metal is unpopular with diehard metalheads, if you keep looking, you'll find many modern metal bands that are surprisingly good, like Edguy and Children of Bodom. Metal did NOT die in the 80s as many think, and this is why it will remain to please us all with its wonderful sound, unlike rap and pop that have no variety or talent whatsoever.