How do the Tora forks compare to each other? I see 302, 318, SL, etc... what do they all mean, and how are they different?

EDIT - PS, can you guys recommend me a decent 26" front wheel with 20mm axle hub? Cheaper the better, but of course I don't want something that's bottom of the food chain. I currently have Sun Rhyno Lite rims with Deore hubs and I'm a fan of them. Something comparable would be nice... It's just hard finding a single wheel. It's like buying the left shoe without the right...