A genre of music that is simply too diverse to define correctly.
Not all metal is aggressive, not all metal is fast, not all metal is about killing, not all metal is about Satan, and ALL metal is talented sophisticated music (save for the bands that just suck).
Metal has to be split up into multiple sub-genres because of how diverse it is.
Heavy Metal - generally refers to late 70s/early 80s bands that have their sound rooted in hard rock. Can also describe bands that don't really fit into any other sub-genre. Black Sabbath, Judas Priest, and Iron Maiden are the big 3 bands of heavy metal, but earlier hard rock bands like Led Zeppelin and Deep Purple are sometimes called heavy metal.
Speed Metal - a type of heavy metal that is faster and can have influences of punk in their rhythms. Not many bands are exclusively referred to as speed metal, because speed metal is hardly distinguishable from thrash metal. Motorhead and Venom, two British bands, are sometimes regarded as definitive of the style.
Thrash Metal - this happened after the New Wave of British Heavy Metal when American bands started to combine it with elements of hardcore punk (there are many other genres to combine punk with metal as well). Bands in this genre play fast riffs with a lot of palm muting and the guitars are often tuned to standard E or standard E flat; many of these guitarists are trained musicians and guitar solos are prevalent. Vocalists can sing (generally in a virtuoso style) shout, or in some cases, growl, generally dealing with lyrical content about war, violence, society, etc. The "big 4" of thrash are Slayer, Metallica, Megadeth, and Anthrax, with other popular American bands being Overkill, Exodus, and Testament.
Power Metal - like thrash metal, it is a derivative of speed metal, though power metal rarely has as much aggression as thrash metal. Power metal makes use of fast tempos, operatic vocals, neo-classical guitar solos, keyboards, and has an "epic" feel with lyrics usually dealing with fantasy subjects. Dragonforce is a popular power metal band, but they are dishonest, and not as original as Sonata Arctica, Manowar, or Helloween.
Death Metal - a derivative of thrash metal that started when bands began experimenting with growled vocals, and more violent playing. Bands who play this style are dismissed by the mainstream as being "talentless" or "just noise", but anyone who studies music will tell you that most death metal is in fact quite musical and very challenging. Lyrics, more often than not, deal with death and gore, but some bands even sing about philosophy, politics, or abstract material. Possessed and Death are credited as being some of the originators, as well as lesser known bands like Master. Though these bands were more rooted in thrash, Morbid Angel came along and really made the genre what it is. Sub-styles of death metal are brutal death metal, which focuses on further extreme and technical playing, technical death metal, which focuses almost purely on playing with as much skill and proficiency as possible (many of them are uncreative), and melodic death metal, which originated in Sweden and focuses on harmonized minor-key melodies.
Black Metal - another derivative of thrash, which has since broken away from it, that usually employs fast playing, and rasped and/or screamed vocals. Many bands make use of lo-fi recording, and melody is often a strong musical focus. The style of vocals used in black metal contains a lot of emotion, and creates the effect that the performer is experiencing strong pain or sorrow, though many vocalists just want to sound evil. Lyrics primarily deal with the occult and Satan, though like all metal, can cover a wide variety of topics. Venom is seen as a key influence on the genre due to the aesthetic, and similarity to first-wave artists such as Bathory. Mayhem became the leaders of the second-wave Norwegian scene.
Doom Metal - inspired largely by the early work of Black Sabbath, and is characterized by slower tempos, chugging guitar riffs, and clean vocals with an "epic" feel. Pentagram and Trouble are seen as key innovators of the style. Doom metal has diversified into death/doom, which contains the growled vocals of death metal and focuses on melody and melancholy, giving rise to gothic metal, and funeral doom metal, which like death/doom usually employs growled vocals and generally uses very slow tempos, repetitive and hypnotic song structures, and ultimate heaviness; Katatonia is an example of death/doom and Shape of Despair is an example of funeral doom.
Folk Metal - mixes black metal or power metal with traditional folk music (usually European folk music). It is often ridiculed for being cheesy, but once you get into it, it's some of the catchiest most fun metal to listen to. Often referred to as viking metal. Ensiferum, Finntroll, Borknagar, Wintersun, and Korpiklaani are all examples of folk metal.
Progressive or Avant-garde Metal - bands in this genre focus on unconventional song structures, and elements that are uncommon in metal or even in rock. Dream Theater is one of the biggest prog metal bands.
Sludge Metal - fuses elements of doom metal and hardcore punk. Recent metal trends have shown that these bands have started to experiment with post-rock and progressive metal in their sound. Acid Bath, Isis, and Mastodon are all examples of sludge metal.
Metalcore - another punk fusion genre, that mixes the modern American hardcore scene, usually with melodic death metal. Some bands focus too much on modern hardcore and are deemed not metal by serious metalheads. Trivium, As I Lay Dying, All That Remains, and Shadows Fall are generally regarded as mostly metal, though bands like Atreyu and Avenged Sevenfold are not. Deathcore is an offshoot of this genre where bands incorporated brutal or technical death metal into their sound, and have been very poorly received by critics and metalheads alike.
Nu metal is NOT a sub-genre of metal, due to having more in common with alternative rock, grunge, or even hip-hop than metal.
Heavy metal is my favorite kind of music, and there are always new twists and turns with unique bands waiting for me to discover.