Originally Posted by Wikipedia

Black metal is an extreme subgenre of heavy metal music. Common traits include fast tempos, shrieked vocals, highly distorted guitars played with tremolo picking, blast beat drumming, raw recording, and unconventional song structure.
During the 1980s, several thrash metal bands formed a prototype for black metal. This so-called "first wave" included bands such as Venom, Bathory, Hellhammer and Celtic Frost.[1] A "second wave" arose in the early 1990s, spearheaded by Norwegian bands such as Mayhem, Burzum, Darkthrone, Immortal and Emperor. The music of the early Norwegian black metal scene became a distinct genre.
Black metal has often been met with hostility from mainstream culture, mainly due to the misanthropic and anti-Christian standpoint of many artists. Moreover, several of the genre's pioneers have been linked with church burnings and murder. For these reasons and others, black metal is usually seen as an underground form of music. Additionally some have been linked to neo-Nazism, however most black metal fans and most prominent black metal musicians reject Nazi ideology and oppose its influence on the black metal subculture.[2][3][4][5]

Originally Posted by RYM

Black Metal is a sub-genre of Metal typified by its usually raw or under-produced sound and Satanic/rebellious aesthetic, as well as simplistic guitar phrases and under-accentuated rhythmic dimensions which allow more power for the atmospheric, detached and wandering riffs. Vocals are often higher pitched than Death Metal (or, "screams" rather than Death Metal's "grunts", speaking in slang terms) and can sound detached or unnervingly direct.

The genre began in the early/mid-80s as an offshoot of the popular Heavy Metal style emerging in Britain during this time, with bands (most notably, Venom with their extremely important Black Metal album in 1982, which most people consider the beginning of the term itself) from this area utilizing powerful occult imagery and a rawer, more stripped-down sound than was usual for typical bands of the times, but other factions of influential bands occurred sporadically in other countries in Europe, including Switzerland, where Hellhammer were concocting their own sinister combination of (what would come to be known as) Black/Doom Metal/Death/Thrash Metal, possibly one of the world's first "extreme Metal" bands (a term for Metal that is not overtly/intentionally melodic in a traditional sense). Over time, the influences culminated in Norway and the other Scandinavian countries where "Black Metal" would come into its own and develop a following greater than when it was in its embryonic stages. It was in Norway and Sweden, with bands the likes of Mayhem, Bathory and Burzum, where Black Metal originated its "northern", cold, isolated sounds which became a staple sound of the "second wave" which extended until around the mid-90s.

Modern Black Metal has diverged quite a bit from its original path. Some see this as evolution and others as regression or even a falling-out from Black Metal totally. The genre is very much a worldwide phenomenon currently, with prominent acts located in Germany, France, Poland, Russia and even the United States (much to the apparent dismay of many fans of the "original" Black Metal which was decidedly European). Sub-styles of Black Metal have very recently been designed around expanding the aesthetic of the genre into Industrial, Psychedelic Rock, Ambient, or Folk sectors of music.