Often misunderstood, over-steer and under-steer are both not as complicated as you might expect. Their name implies what actually occurs.
Over-steer occurs when the rear end pushes out, resulting in the car turning to a larger degree than expected, and often resulting in spinning out.
Under-steer is just the opposite. The car turns to a lower degree than expected and the vehicle takes a trajectory that is straighter than expected.
The causes of under-steer and over-steer are much more complicated. Weight, power, chassis set up, driving style and many many other factors all combine to result in under-steer or over-steer. A perfectly built car will not have a tendency towards one or the other, but this is almost impossible to actually achieve. The trick being, a driver must know their car, and drive it to it’s strengths, while also anticipating the under-steer or over-steer and taking a line what plays to that end.