I think it was Cale Yarborough who said, "[Understeer] is when you hit the wall with the front of the car. [Oversteer] is when you hit it with the back of the car."
Understeer is when the car wants to plow through a turn and not change direction. Oversteer is where the rear-end wants to overtake the front. On the street, that usually leads to fishtailling.
(Note to nitpickers: He actually said "tight" and "loose", which is why I put "understeer" and "oversteer" in brackets.
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