Prator: I'm not sure there is any functional difference, actually. I'm looking through the lists of games I own, and how the "soundtrack" download is labeled, and it seems to be slightly different for every game. Yes, many are labeled "soundtrack" or "in-game soundtrack" or "[game title] soundtrack," but I cannot see a definite pattern to the naming system.
For instance, "UFO tracks" has some of the musical scores from UFO: Aftermath, but not the ambient music tracks that play during actual missions. The "soundtrack" from Giants: Citizen Kabuto has ALL of the music that plays during the game, including some incidental things that you only hear during cutscenes, like the British national anthem. "in-game soundtrack" from Earth: 2150 also has every piece of music that plays during the game EXCEPT for incidental cutscene music.
thanks for the info, it pretty much answers everything I was wondering about.