Oversteer, in an automobile, is when the rear tires have a loss of traction during a cornering situation, thus, causing the rear of the car to head towards the outside of the corner.
The act of deliberately sending a car sideways through a series of corners is actually a popular form of motor sport in Japan known as drifting. A more technically correct definition is that oversteer is the condition when the slip angle of the rear tires exceeds that of the front tires.
Rear wheel drive cars are generally more prone to oversteer, in particular when applying power in a tight corner. Contrary to popular opinion, modern rear wheel drive cars are much more user-friendly in this regard as they are set up to understeer and the more powerful ones even have on-board computer systems which can automatically brake the car or override the driver's throttle inputs. This is because understeer is generally much safer for novice drivers, whereas oversteer is much more difficult to correct when one's not prepared for it.
The natural reaction of most drivers in case of loss of control is to try to slow down - either by lifting their foot off the gas pedal or even by braking. Both of these will help bring an understeering vehicle under control but can have disastrous effects in the case of oversteer. This is because braking causes weight transfer towards the front of the car, thus reducing rear traction even further. The correct oversteer correction is to gently steer into the slide then take the power away as needed. Indeed, cutting the power mid-corner can induce oversteer even in a front wheel drive vehicle. This is known as lift-off oversteer.
A car that tends to neither oversteer nor understeer when pushed to the limit is said to have neutral handling. Most racing drivers prefer their cars set up just on the oversteer side of neutral. This is almost always the fastest set up as it helps turn the car around tight corners.