The fundamental principle of efficient cornering is the 'traction circle.' The tyres of a racing car have only a finite amount of grip to deliver. This can be the longitudinal grip of braking and acceleration, the lateral grip of cornering or - most likely in bends - a combination of the two. Racing drivers overlap the different phases of braking, turning and applying power to try and make the tyre work as hard as possible for as long as possible. It's the skilful exploitation of this overlap, releasing the brakes and feeding in the throttle to just the right degree not to overwhelm the available grip, which is making the best use of the 'traction circle'. The very best are those who can extract the maximum amount from the tyres for as long as possible.
Oversteer and understeer are vital to understanding the way a car corners. They refer simply to the question of which end of the car runs out of grip first. In an understeer situation the front end breaks free first, the car running wide as centrifugal force takes over. Oversteer is where the back end of the car loses adhesion and tries to overtake the front - think in terms of a road car's 'handbrake skid'.
Understeer is inherently stable - once the car reduces speed sufficiently grip will be restored, which is why almost all road cars are set up to understeer at the limit of adhesion. But it also slows a car, which is why Formula 1 chassis engineers try to avoid it. Oversteer is, by contrast, highly unstable. Unless a driver acts to correct it quickly with skilful use of steering and throttle it can result in a spin. But an 'oversteery' chassis helps the driver to turn into a corner and, at the limit of adhesion, enables a skilled driver to carry far more speed through a corner than understeer. Which is why, to a greater or lesser extent, all F1 cars are set up with an oversteer characteristic.
A racing car takes a corner in three stages - turn-in, apex and exit. Turn-in is, like it sounds, the broad term given to pointing the car into the corner. Weight transfer under braking, moving the effective mass of the car from the back axle to the front, encourages oversteer during this phase, which the driver will use to help make the turn. The apex or 'clipping' point is the corner's neutral point, the place where the transition between entry and exit is made. Different corners may have different natural apexes, whether early or late (before or after the mid-point of the corner), and individual drivers may also use different apexes according to their personal technique. (A late apex can allow power to be applied earlier and can help to 'straighten out' the corner). And the exit phase is where the driver will blend the throttle back in as the steering is progressively wound off: ideally keeping the car right on the edge of the traction circle through an acute sense of balance.
Many things can affect the traction circle, including a car’s downforce level (an aerodynamically efficient car often appear as if it corners on rails), the amount of grip yielded by the track surface (which is dramatically reduced in wet or dirty conditions), and even the subtle changes in the camber of the road (its side-on gradient). The most successful drivers are consistently those who are best at judging the limits they can take their cars to under cornering - and who go there as often as possible.