Since Tolkien basically invented orks, elves (as we know them), ents, etc...he\'s the place to look for this. Orks are a corruption of elves by Melkor (saurons boss). I believe the difference between orks and goblins is that goblins are basically smaller ferrel orks, where orks as we know them only exist as an organized army of Mordor (and later under Saruman). Uruk-Hai are black orks created by Saruman, by breading orks with hill men - ewwww. They were larger, and much less senstive to sunlight. Similarly Olog-Hai are black trolls, (imagine that breading!) which would not turn to stone in the daylight.
Now GW has a COMPLETELY different idea. Orks being fungus, which spread spores to reproduce, and it works basically as Chrispy layed out.