I'm surprised no one has mentioned this yet. Not the 5 year old version, but very relevant.

There are two types of friction here that apply, kinetic friction and static friction. Static friction is the friction between two objects not moving relative to each other. It is what holds still things in place on slanted surfaces. Kinetic friction is the friction between 2 objects, at least one of which is moving relative to the other.

Static friction is almost always greater than kinetic friction. Meaning the force holding an object still is greater than the force slowing a moving object down.

Now, the beauty of a wheel, from a physics standpoint, is that because of the rolling motion of a wheel, the touching surface of the wheel is using static friction rather than kinetic. When using friction to create speed (which is what driving and walking both do) this makes wheels faster. A point on a wheel that is not skidding will contact the ground, and remain touching that SAME point of ground as the wheel turns, lifting other points from the ground and adding new ones to it. I suck at illustrating or i'd do so. Best i can do is to point out that when running, each step plants on the same point of ground as you move forward, a wheel is like that but with basically infinite different "feet".

Now, when slowing down, the same principle applies. Since friction is what slows down an object, the idea of an ABS system is to maximize friction. A tire that is skidding is no longer functioning as a wheel, and the "point" in question on the wheel is no longer staying still relative to the ground, but is moving along it. this means the wheel is now using the lower co efficient of kinetic friction rather than the higher one of static friction. Anti lock brakes aim to keep the wheel rolling while applying the maximum pressure (which, in turn, creates the maximum friction) possible while still using the higher coefficient.

So basically, the ABS system allows the wheels to slip over the breaks, so they maintain maximum friction on the road, which increases both the rate of deceleration and the general maneuverability of the car.

Anyone who tells you ABS is less safe because it takes control from the user simply does not understand the physics involved.

Edit Also note that anyone talking about the system basically pumping your brakes on and off 10 times a second are misunderstanding the physics of it as well.