Sin is short for Assassin, one of the second jobs for the thief and the second most popular class in all of Maple Story next to mages. Personally I find most sins to be obnoxious jerks, but I'm primarily a mage, and I think most sins tend to feel the same way about mages =D
One of the best sites out there is
http://www.hidden-street.net They have Weapon and Armor stats, info on all the quests and monsters, and in general good information. They also have a lot of guides, which are useful as guidelines in figuring out where to put AP and SP.
As for thief moves... The two second jobs are vastly different, so it's probably a good idea to have in mind which you'd like to be when putting in skill points. Bandits use daggers, while Sins use Claws and throwing stars. Bandits are a lot like warriors in fighting style, but they're some of the strongest characters around in 3rd job. Sins are long range and tend to jump around a lot. If you're going to be a bandit, max Double Stab, if you're going for a sin, Lucky 7 and Keen Eyes. Generally if you're going for one class, you don't max the other moves, but it's not that big a deal I think. As for the other moves, it's up to you. Personally I think Dark Sight is rather fun...
If you're going to do a mage right, you kinda have to put up with sucking until around level 17 or so, as you should max Max MP Increase before anything else, other then one point in bolt. After that it's good to max MP Recovery, so you can actually fight with some endurance. But once you get those two covered, you can start maxing your attack, and once that's covered, you're going to have a lot of fun. First 8 levels on a 4/4 are painful though T-T
Hope that's helpful and not ranty. There should be a warning not to get Joey started on Maple...