Metal originally was one genre. But as time passed more genres were born. it is in the 80's, during the days of NWOBHM and Thrash Metal (two very awesome and popular metal genrews) that metal started changing noticeably.
Thrash metal was born when NWOBHM and Hardcore Punk somehow got combined. The first extreme metal genre was created, one that had the energy, speed, craziness and politicalness of punk and the complex riffing, complex lyrical writing abilities, guitar virtuosity and instrumental pwnage of NWOBHM. Thrash was perhaps the most popular and successful genre of metal during its years, with bands like Megadeth, Metallica, Slayer and Anthrax (the big 4 of thrash) as well as smaller acts like OverKill, Testament and Annihaltor enjoying much success. Unfortunately, after the 80's and the appearence of grunge, thrash started fading away and bands simply started lacking the energy and creativity of their past. From what is known, thrash metal though, is slowly getting back onto its feet and coming back with a vengeance.
In 1980's, after Venom made their monumental "Black Metal" album, black metal was (sort of) born, although it was not THAT well defined yet.
Bathory, Celtic Forst and Mercyful Fate (note: those bands were not really black metal, but they came really, really close.) then came along and helped solidify black metal, defining its form and giving black metal shape. (Black metal often spoke of the occult, Satan, blasphemy, nihlism but sometimes focused on fantasy and folklore, and even national socialism. Their was usually a very fast drumbeat, an often equally fast guitar and bass, shrieky/raspy vocals and sometimes keyboards or symphonic elements, just to add atmosphere) Then the more commonly associated with black metal bands popped up, such as Darkthrone, Mayhem, Burzum and such. Black metal, despite its age and the difficulty of getting use to it, remains quite popular in underground extremem metal circles. Was very popular in the 1990's. As of right now, the genre seems to be doing okay, with no really big problems. Some random black metal bands: Antestor, Satyricon, Emperor, (early) Behemoth, Dark Funeral, Enslaved, Old Man's Child and Solefald.
Death metal also popped up during this time, it being the "spawn" of thrash metal. The first death metal bands were like thrash bands... on steroids. They took all the characteristics of thrash and pushed them to the next level, resulting in incredible amounts of heaviness and loudness. The early history of death metal is rather fuzzy, but it is presumably Possessed who made the first death metal album and fellow bands like Death who laid down the blueprints. And so, having its shape defined, death metal went on a musical rampage and mutated itself into a gamut of genres and fusion genres, which include melodic death metal, technical death metal, as well as deathcore and presumably grindcore (which was presumably the doing of British metallers Napalm Death). It remains a very influential genre to this day, although there are some problems of a possible commercialization of this genre in progress, mainly concering melodeath and deathcore. Good examples of random death metal bands are Cannibal Corpse, Mortification, In Flames (melodeath), Cryptopsy, Nile, Obituary and Cynic.
Of course, there are melodic metal genres too.
Power Metal popped up in the 80's too. It was focused mainly in Europe, with some bands popping up the U.S. It tended to be a rather happy and upbeat genre. It mainly incorporated characteristics of traditional heavy metal, but would often add bits of thrash or speed, as well as create an Epic feel. Usually fast paced, with clean vocals, fantasyish lyrics (includes cosmology, metaphysics, religion, mythology etc.), powerful, uplifting choruses, high-register clean vocals, speedy guitars and keyboards often, as well as the occasional symphonic elements. Has not changed too much and the genre does not seem to be suffering from as many problems as other genres. Good examples of power metal include Blind Guardian, Helloween, Stratovarius, Jacob's Dream, Sonata Arctica, Angra and Rhapsody of Fire.
Progressive Metal: A very technical and complex genre of metal, characterised by intricate compositional structures, odd time signatures, intricate instrument playing, long songs, lyrical epics and concept albums. Has not really changed that much or been negatively affected a lot either. Dream Theater, possibly the most skilled, well known and most popular prog. metal band of today, along with Queensryche and Fates Warning (and maybe Symphony X) defined this genre. Good examples of prog. metal are Shadow Gallery, Opeth, Ayreon, King's X and Pain of Salvation.
Doom Metal - The 2nd oldest genre of metal. Also the slowest. Tends to be of a pessimistic nature and often evokes an atmosphere of darkness, despair and misery. Strongly influenced by Black Sabbath. Guitars and riffing tend to be downtuned and loud. Newer bands tend to use keyboards. Tempos are rarely fast, and usually are mid-tempo or slow. Vocals are usually clean, although some genres make use of growls or shrieks. Examples of doom bands are Paradise Lost, Crowbar, Amorphis, early Anathema, Candlemass, Dolorian and Esoteric.
Nu-metal: Not a true metal genre. Actually a form of hard rock. Has similar characteristics of metal, but they tend to be all watered down. Incorporates elements of rap, grunge, alt. metal, electronic music with thrash or groove metal, although the overall result is still hard rock. Guitar playing tends to be lacking in complexity and usually used to create emotions of angst or hostility (to move people to the beat), consisting usually of a few notes and power chords played in rhythmic patterns. Drums are often in the form of hip hop and funk influenced beats and bass, an important nu-metal instrument, tends to be reminiscent of hip-hop or funk. Sometimes, there are even turntables. Vocals bear ressemblance to rap, punk, pop, normal rock and such. Lyrics tend to be angsty, concerning life struggles (eg. childhood abuse, alienation, relationship struggles etc.) as well just everyday life. nu-metal (also known as mallcore) tends to be hated and despised by many metalheads. Some examples are Korn, Limp Bizkit, Spineshank, Disturbed and Linkin Park.
Metalcore- A fusion of hardcore punk and metal, resulting in a genre which is not either. Like thrash, it has the speed of hardcore with the instrumental playing style of metal. Also, it is notorious for its sometimes incredibly cheesey/emo-like breakdowns. Bands tend to have either more metal influence (Trivium, God Forbid, Killswitch Engage etc.) or more hardcore punk influence (Zao, Norma Jean, Bury your dead etc.). Usually, if the influence is metal influence, it tends to be melodic death metal and melodeath influenced bands are sometimes known as swede-core or gothencore. Lyrically, focuses on inner/moral strength, integrity, life-struggles and such. Also known as mall metal and like nu-metal, is a very controversial genre, hated by some, loved by others or simply not minded. Examples include Zao, Norma Jean, As I Lay Dying, Trivium and Killswitch Engage.
Nu-metal and metalcore ain't metal
It's an encylopedia of all things metal.