Black metal is a subgenre of metal typified by its raw, often under-produced sound and Satanic or rebellious aesthetic, as well as simplistic guitar phrases and under-accentuated rhythms which allow for more emphasis on the atmospheric, detached and wandering riffs. Vocals are often higher pitched than in death metal (or, "screams" rather than death metal's "grunts") and can sound either detached or unnervingly direct.
The genre began in the early to mid 1980s as an offshoot of what is known as the New Wave of British Heavy Metal, with some bands of the movement (most notably Venom with their landmark album Black Metal in 1982, which most people consider the origin of the term itself) adopting occult imagery and a rawer, more minimalistic sound than was usual for bands of the time. Other factions of influential bands occurred sporadically in other European countries, including Switzerland, where Hellhammer created their own combination of black, doom, death and thrash metal. Over time, the influences culminated in Norway and other Scandinavian countries where bands such as Mayhem, Bathory and Burzum developed a "northern", cold and isolated sound. This sound would become a staple of the "second wave" of black metal, which extended until the mid 1990s.
Modern black metal has diverged significantly 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 at present, with prominent acts located in Germany, France, Poland, Russia, and even the United States (much to the dismay of many fans of the "original" black metal, which was decidedly European). Sub-styles of black metal have recently expanded the aesthetic of the genre to include elements of industrial, psychedelic, ambient or folk music.