Characterized (though not always) by drop-d tuning, 5/4 time, and a notable absence or minimalist approach to vocals, Post-Metal focuses on riffing and build-up to that sonic darkness that hearkens back to old Black Sabbath or Judas Priest. It's generally considered the "thinking man's" metal.
It avoids emphasizing the typical metal aesthetic: the dark, somber grimaces, the emphasis on guttural vocals that often overpower other instruments. Not to say that these ascriptions don't exist in post-metal, they're just more a byproduct than a manufactured assertion.
Post Metal seems to be about building momentum with music, getting to the immense wall of sound that eventually grinds your ass to powder. But not before some serious seduction first.