A torsion bar is a type of spring (equivalent to coil and leaf springs) to support the weight of the vehicle. Each type has their own characteristics. Torsion bars can take up less space in the chassis (and, of course, can adjust the ride height).
Torsion bars can also be hollow to save weight. The spring rate of same size hollow vs solid bars is not exactly the same. The larger the bar (OD), the more resistance to torsional twist.
More info here. Much more if you continue a search in Google:
http://www.off-road.com/chevy/tech/2003_08/suspension.html
Anti-roll (aka sway) bars also use the torsion bar effect (twist) to reduce body roll and control weight transfer during cornering to improve and/or effect handling.
Hope this helps,
Sherwood
http://members.rennlist.org/911pcars