Oversteer, or what race car drivers call “loose”, refers to the back end of a car breaking traction and sliding sideways. This usually happens quite suddenly, and if not corrected properly will generally result in a spin.
In rear wheel drive cars, oversteer is often caused by a lack of traction at the rear, combined with abrupt steering, braking or sudden application of power to the rear wheels. Many things can cause oversteer, but slippery conditions, jerky steering wheel movements and inappropriate braking are usually the root cause.
Anyway, now that you know about oversteer, what do you do to correct it? As with understeer, slightly less of what you're doing. Gently release some pressure from the brake or gas pedal, whichever your foot is on. Sudden movements will make things worse. Now, steer the wheel (smoothly) in the direction that the back end is coming around. You’ll feel the car stop sliding as the rear tires regain traction.