Understeer is where you hit the wall with your front end. Oversteer is where you hit with your back end.
Understeer is generally more preferable from a loss of control standpoint, because you see where you're going and are in an easier position to regain/maintain control. I think it also scrubs off more speed. The wide majority of street cars are designed with balance toward understeering. Oversteer in a front-driver is generally an out-of-control condition, since we can't use the throttle to control the amount of oversteer angle, at least not as readily as a rear-driver can. A really good driver in a FWD car can toss intentional oversteer and use it to his advantage, but in most cases, bad idea.
