To the poster who said he didn't know how to classify or define Post-Rock - that's a good point.
I guess it applies to any instrumental rock band or mostly instrumental rock band starting from the late 80s, early 90s. I actually first heard the term as used to describe the music of Toirtoise and they're indeed pioneers. The thing is Tortoise really has little in common with EITS except that it's instrumental!
But being a fan of 70s Progressive Rock, I think that label applies to these groups along with bands like Radiohead. These are bands that at the core play "rock" music but outside of standard rock music conventions: non-standard tempos, experimenting with sounds, creating music with a wide emotional, musical and lyrical range - music that often sounds timeless.
Listen to the music of any of these bands. You don't necessarily hear the influence of it's time - in other words none of them have an "80s", "90s" or "00s" sound. They could have been recorded 20 years ago or 20 years from now.
A perfect example - and for those willing to expand their musical appreciation from the "post-rock" of this topic and into early "progressive rock" - is Genesis' 1974 double album, "The Lamb Lies Down On Broadway".
This album sounds like it could have been recorded last month and it's 32 years old. It features very high musicianship yet is very musical and somewhat accessible (as accessible as a double concept album can be!) and features some of the sound experimentation and atmospheric instrumental playing that is recognizable in today's "post-rock".