According to Wikiedia, post-rock is a subgenre of rock music characterized by the influence and use of instruments commonly associated with rock, but using rhythms and "guitars as facilitators of timbre and textures" not traditionally found in rock. Post-rock bands are often without vocals.
In 1996, Scot Hacker wrote in The Utne Reader, "When modernity came to an end and folded back in on itself, rock sputtered out in an uninspired tailspin, leaving a trail of a thousand identical grunge bands to kick hairballs around in the garage. The past five years of tiresome, lo-fi grunge has unflaggingly rehashed punk’s once-definitive assault, but without conviction. In the wake of rock’s last gleaming, a handful of seekers forge something new where nothing new is possible. The only way to come to sensible grips with the alleged phenomenon of 'post-rock' is to consider that it is to rock as post-modern is to modern. Post-rock bails out on the world of rock and goes 'stratmospheric,' leaving behind the posturing, the testosterone, and the formulae that defined pop/rock in all its forms for decades. In post-rock, the accreted layers of musical history form a substrate through which artists speak of the mysterious present. Remixology becomes more than a collage technique – it’s the lingua franca, the currency of all that is good after rock gives up its pathetic struggle to rebel (against what? The alternative has been subsumed by 'the man.')"
The Japanese experimental band Boris (ボリス) are widely considered to play predominantly in the metal genre, although the band has also been labeled as noise rock, drone, doom metal, psychedelic rock, dream pop, shoegaze, drone metal, ambient and even J-pop. Their 2005 album, Dronevil, their eighth, consists of a double album with the first LP, Drone, being quieter and falling more within the ambient/drone spectrum of Boris' work, and the second LP, Evil, more guitar oriented and falling into the doom/metal spectrum of their work. Though the albums can be heard separately, the original intention is for both LPs to be played simultaneously through individual record players and speakers.
The album's cannibalism of various rock genres and accreted layers of sludgy substrate sound to this listener closer to Hacker's definition of post-rock than anything else, with compositions that should simultaneously both enthrall and offend the modernist and the metal-head alike.