Fortunately, a well designed car does not suddenly break into a skid. As the tyre approach their traction limits, they tend to slip sideways across the road. The angle between the tyres actual path and its natural path is called its slip angle. This gives the driver advance warning that the front/rear of the car is in danger of breaking loose and starting a skid.
If the rear tyres approach their traction limit more rapidly than the front, then the effect is for the rear of the car to steer a wider path than the front wheels. This rotates the car more than the driver intended and, if nothing is done, leads to the car turning a smaller radius corner. When this occurs the car is said to oversteer.
If the front tyre approach the traction limit more rapidly, the effect is that the front of the car takes a wider radius curve than the driver intended. The car is said to understeer.
Understeer is safer than oversteer. If the car understeers, and no correction is made the result is a wider corner than intended, but the car remains stable. If the car oversteers, the turn made has smaller radius than intended. The smaller radius produces higher cornering forces bring the required traction even closer to the limit of the rear wheels, and thus causing even more oversteer. The situation becomes worse until the rear wheels lose grip completely; the car spins and all directional control is lost.