Torsion bars (or torsion rods) deserve their own section because they are a type of spring which can be used in place of coil- or leaf-springs. It's one of the topics I get the most e-mail on, so instead of continually sending the same answer, I thought I'd cover it on this page.
A torsion bar is a solid bar of steel which is connected to the car chassis at one end, and free to move at the other end. They can be mounted across the car (transverse like the rear suspension on the Peugeot 205 and Renault 16) or along the car (longitudinal, like the front suspension on the Morris Minor) - one for each side of the suspension. The springing motion is provided by the metal bar's resistance to twisting. To over-simplify, stick your arm out straight and get someone to twist your wrist. Presuming that your mate doesn't snap your wrist, at a certain point, resistance in your arm (and pain) will cause you to twist your wrist back the other way. That is the principle of a torsion bar.
Torsion bars are normally locked to the chassis and the suspension parts with splined ends. This allows them to be removed, twisted round a few splines and re-inserted, which can be used to raise or lower a car, or to compensate for the natural 'sag' of a suspension system over time. They can be connected to just about any type of suspension system listed on this page.
The rendering below shows an example longitudinal torsion bar. The small lever at the far end of the torsion bar would be attached solidly to the frame to provide the fixed end. The torsion bar itself fits into that lever and the suspension arm at the front through splined holes. As the suspension at the front moves upwards, the bar twists along its length providing the springing motion. I've left the shock absorber assembly out of this rendering for clarity.