I'll try my hand at distinguishing between Prog and Post rock.
Firstly, we must not forget that Post rock is strongly influenced by Progressive rock which, by necessity, makes Post rock and Prog rock share several musical elements.
However, to listen to both genres, one detects a distinct difference. While Progressive rock likes to play with conventional song structure, and draws much of its musical inspiration from jazz and freeform styles of music, Post rock seems to (by and large, I recognize that there are exceptions) take its melodic inspiration from later styles, like punk (and its offshoots), metal, and shoegazer. Progressive rock, while undoubtedly branching off from conventional rock, tends to use modest guitar tones, which put an emphasis on the clarity of the guitar sound. This is mostly because Prog focuses on complex, difficult styles of guitar playing, that are meant to be representative of the player's ability. Post rock does not necessarily use complex guitarwork - in fact, most bands of the genre use a riff structure, as opposed to the solo structure approach of Prog. Additionally, virtually all Post rock bands make heavy use of guitar effects, and stray to the emotive side of the scale (in effect, sacrificing complexity for emotional atmosphere).
Granted, these differences may seem semantic and picky, but the difference is quite obvious to anyone who devotes time listening to both genres.
Hope that helps.
