Understeer is when the car doesn't turn in as much as you would like it to when coming into and out of turns. The front tires "plow" and you don't get the sharp response you're looking for.
Oversteer is the opposite of that, meaning the rear end comes out more than you want it to.
Both can be adjusted with camber, tires and suspension components. It just want you want the car to do that will determine what to buy and install.