A coil spring is simply a long metal bar that's been bent into a slightly flexible coil that looks like the railing on a spiral staircase. There are actually two types of coil springs -- tension coil springs and compression coil springs. Tension coil springs are built to resist stretching and work a lot like a bungee cord. A Slinky, in fact, is technically a tension coil spring; if you stretch a Slinky apart, it wants to pull itself back in.
The type of coil spring used for steering control is a compression coil spring. This kind of spring resists compression, so that when you push down on it in an attempt to keep all the metal parts together, the spring bounces back into place the moment you let go. Coil spring steering stabilizers help control the steering by keeping a "memory" of the wheels' center position. When you turn your steering wheel, the coil spring is pushed together. If you let go of the steering wheel, the tension in the coil spring will release, and the steering wheel will return to its original location.
How do coil-hydraulic steering stabilizers differ from the coil spring design? Read about the extra juice hydraulics give to traditional springs on the next page.