This happens when the front of the car does not have as much grip as the driver wants, and the car has a tendency to plough straight on at a corner. It wears out the front tyres and slows the car down.
The car can be adjusted to try to cancel it out, usually by increasing the amount of downforce at the front, or changing the suspension settings.
On the track, the only real way to deal with it is to enter the corner at a slower speed, although some drivers can find ways to minimise its effects.
Drivers are very rarely completely happy with the handling of their cars - by definition, a car being driven on its limit will have some problems.
However, they can be happier sometimes than others. This will be when the behaviour of the car allows the driver to use it in exactly the way he wants.
The end result is that he turns into the corner at the fastest possible speed with all four wheels of the car on the edge of adhesion sliding the same amount as each other.
This sets the car up for the maximum speed down the following straight and, eventually, the optimum lap time.
This happens when the front of the car has more grip than the rear. It creates a feeling that the car is trying to spin.
Some drivers like oversteer in limited amounts because it can make the car feel more responsive, but it carries the risk that the driver will lose control.
It can be dialled out using the settings of the car in the opposite way from which understeer would be cured.