Oversteer and Understeer

Tire pressures can be used to help correct a handling problem, such as a tendency to oversteer or understeer.

Oversteer.

When oversteer occurs, the back tends to swing around, sending the car into a spin. It is more likely in a rear wheel drive car and increases in likelihood if the car has a rear engine, as this raises the weight carried by the rear tires.
In both oversteer and understeer, the tires are losing their ability to grip the road surface. This usually happens because the speed and weight of the car is exerting a force on the tires and deforming it so that part of the tire is no longer in contact with the road. Slowing down and steering into the slide will correct the situation, but slow gradually, do not apply the brakes.
To correct oversteer with tire pressures, raise the rear tires by 7-14kpa(1-2lb/in²), while maintaining front tire pressures.

Understeer.

This can occur when you drive a car rapidly around a corner. The front wheels do not turn into the corner as fast as expected and tend to "plough on". This tends to affect front wheel drive cars more. Front tire wear can be increased considerably.
To correct this problem with tire pressures, raise the level in the front tires by at least 14kpa(2lb/in²) and up to 24kpa(3.5lb/in²), while maintaining rear tire pressures.

In both these situations, worn or inappropriate tires can contribute markedly. Match the tires you buy to the driving you expect to do.