Black metal is a musical genre which emerged in the early 1980s predating the great expansion of heavy metal 'extreme' genres. It evolved out of thrash metal, like death metal.
There are two views on the genre. One views black metal as a very specific form of music that must adhere to a particular 'style' in order for it to be a part of the genre; while the second considers the lyrical and philosophical/political ideology of the music (likely to be heavily inspired by Friedrich Nietzsche) to take more precedence in defining the genre itself, rather than 'style'.

Source: http://en.wikipedia.org/wiki/Black_metal
Read on Death Metal: http://en.wikipedia.org/wiki/Death_metal