There are many different makeres of SKSs, but most of them that are on the market currently are Yuglslavian made, called a Zastava 59/66. This is a minor variation on the SKS which added a grenade launcher and a little gas tube valve that allowed for shooting the grenades with a special blank cartridge.
The two biggest complaints with the surplus Yugos is that the barrels are not chrome-lined and the gas system valve. Both items suffered during their military service with the use of corrosive ammo and the lack of regular cleaning. The barrel that you get on a $100 Yugo might be pitted, or it might have trouble cycling properly because of the gas system.
The reality is, though, that the 59/66 is a great deal right now, and the supply is drying up. If you get one that has a bum gas system, there are many easy fixes for it, and even a pitted barrel will more than likely be a great shooter.
Don't hesitate to snap one up...and you'll love it!