Basically negative caber is when the top of your wheel is tilted in towards the car (if you're looking at it from the front or back of the car). The knuckle is the part of the suspension which the wheel hub bolts to. If the shop bends it they can correct negative caber to a certain extent. The problem is that now part of your car is bent.

In back you can just space the upper control arm away from the car a bit with washers so it's not too much of a worry. Up front though there's no easy way to adjust things without getting a camber kit. Ingalls is alright although if you have a ton of correction it will sometimes bang against the wheel well liner. Their kit is basically adjustable bolts for the upper A-arm. Otherwise skunk2 makes a set of A-arms with camber adjustment built in to them which is definately nicer, but more expensive.