Hello there.
Here's my point of view on your questions:
1) "OST" means Original Sound Track.. or Official Sound Track.
I like to use this term for Official Releases, that's the way it should be.
But you'll find lot of people naming game rips "OST".. i personally don't like that at all.
2) OST or Game Rip ? This is a hard question and can be debated.
Often.. OST are not really really complete and some tracks can be missing, and you could want a Game Rip to have those missing music.
On the other way OST can have a better sound quality than a game rip (but that depends on lot of things, at least OST have a better sound than an rip made with emulation).
If you can find an official soundtrack that is complete, i guess there is no use to make/ask a game rip.
Finally there is the little difference of "officiality", collectors like me will prefer to have the official releases, something you can put a name on it, a good tracklist, album/music credits, covers.. Something that can be difficult with a game rip.
But the game rip is also needed, there are millions of games without a soundtrack.
3) That is harder than "OST = official".. there is a high chance for the music to be official if the album is called "original sound track", but like i said you'll find lot of names or mislabelled unofficial stuff..
The best way to know if it has been officially released is to check a good Video Game Music database. (like Game Music Revolution).
90% of official soundtracks have a Catalog Number, and you can easily search/find them with it.
Note that official albums can also use lots of different names.. like "Music From", "Inspired By" "Sound Of"..
Game Companies use different Publishers for their soundtracks..
For example, Konami use "King Record", capcom use their own record label "Suleputer".. and the list goes on and on.
This is the place you should look if you search for official releases:
Game Music Revolution - VGM Database (
http://www.gmronline.com/index.asp)