This has been widely debated over the years, and we've touched on it here partly I think, but in the recent Insider (#76) there's an interesting note from LFL about the situation.
Lucas has always said he intended Clones to = Stormtroopers, but some discrepencies in the films, EU, etc., have said otherwise.
Lucas still holds that Clones = Stormies to him, but he's seemingly (or so was said in Insider) backed off that stance somewhat and has himself explained the height and voice differences of teh films away...
Insider says that Lucas feels that new hosts were used (an early fan theory) and thus the variances in height and voices, but also that the process by which hosts are picked is politicized and subsequently less thann "perfect" hosts are chosen. A high ranking officer's nephew is chosen based n the officer's position rather than the young man's personal military skill.
So Lucas is taking a somewhat fan-based stance, ok, cool...
He goes on further to say though that the EU's NOT incorrect and that conscriptions, volunteers, and other sentient humans are within the ranks as well... IE: Kyle Katarn, Davin Felth, etc...
It's interesting to see Lucasfilm themselves back off the idea that "They're Clones, that's what I always intended", which Lucas has more or less claimed, and now that they're trying a more EU friendly approach.
I'm more in tune with the new "answer" as it were... I still debate the validity of cloning at all. Personally I think Clone ranks are thinned down dramatically (my persnal preferential thought oon the matter), and are hardly used anywhere. Deemed "creepy" even by some troops perhaps?
The article's interesting nonetheless.
And irregardless, nobody's answer is canon still... The ranks aren't proven to be either on-film and I highly doubt they are ever actually explained in a canon fashion. Suits me, but it leaves some interesting theories by both fans and Lucasfilm itself.