Oversteer and understeer are terms that describe the way a vehicle turns. Oversteering is when the front wheels turn more than the rear wheels while understeering is when the rear wheels turn more than the front ones. If you're not sure how to tell if your car has oversteer or understeer, this article from Country Chevrolet Inc. in Warrenton, VA will help clear it up!
If you're driving too quickly into a turn, the weight of the vehicle will be shifted to the rear wheels. This causes them to lose grip and slide outwards (oversteer). If you enter a corner too slowly, the weight will shift to the front wheels causing them to lose grip and slide forward (understeer).
So, if your car is oversteering, what do you need to do? There are three things that can be done. You can brake while entering the turn which will reduce speed and allow weight transfer to the front wheels. You can turn into the slide by counter-steering (turning in the opposite direction of the slide).