Before any of the 'extreme' metal genres, there came Thrash.  It was a child of Classic Heavy metal and punk rock.  Guitar riffs were faster here than in any metal previously.  Thrash songs can consist of just a few chords, but they are played very rapidly and with plentiful time changes  It is also necessary for wailing guitar solos to be included.  Truly a pioneer of metal, Thrash was the first genre to use double-bass drumming. For the most part, vocals here are shouted harshly, but can sometimes be melodic for periods of time.  Lyrical content in this genre will often include anger towards society and government as well as occult content.  When most North American people hear the term 'metal', it is probably a Thrash metal band that they think of.  Metallica, Anthrax and Megadeth are hugely popular metal bands and are Thrash. (Except, in the case of Metallica, who was once Thrash).