Post-Rock (post rock, postrock) is an emerging genre of music that has a few characteristic tendencies.

Primarily, I think what it comes down to is an emphasis on making (usually) very beautiful SOUND and TEXTURE than anything else. Obviously, that definition of beautiful changes from band to band.

Some (Low, Impulse Nine) still adhere to song structure and more-or-less standard Western harmony, but that's not the emphasis. Low puts the emphasis on what's between the notes of their slower-than-slow songs. Their version of "Little Drummer Boy" was used on a Gap commercial. Impulse Nine combines genres into textures, using work like NIN's more recent work and Sigur Ros' sonic landscapes into more conventional songs.

On the other hand, godspeed you black emperor! and Sigur Ros has 18-minute (or more) epics that build to a ridiculous crescendo using, basically, theme-and-variation between its six to ten people (depending on the day).

While none of the things on this list are *required*, really, it's normal to have the majority. I listed the attributes with the best example of a band that uses it:

-Mostly instrumental (mogwai)
-Epic (15+ minute) songs (godspeed you black emperor!)
-Ultra-slow pacings (low)
-Unusual ways of playing instruments (Sigur Ros)
-Larger bands (gybe!)
-Sense of massive self-importance (Sigur Ros)
-Very serious-sounding music (any of them)
-Way excessive use of theme-and-variation (gybe!)
-Use of a lot of noise (Magnog)
-Distain for popularity of any sort
-Extreme layering of instruments
-Mixing genres (Impulse Nine)
-Little (if any) use of proper 'melody'

There's sure to be more from this genre, because it's growing fairly rapidly. It's definately 'underground', but as Rolling Stone picked up on Sigur Ros' album as one of the best of '00, it's probably a matter of time when you can tell your friends you read about it here first :)

A few links:
Low -
Mogwai -
godspeed you black emperor!
Impulse Nine -
Sigur Ros -
More Post-Rock Info: