Anti-Roll Bars

An anti-roll bar (sometimes called sway bar) connects two wheels to each other. As the car rolls, one wheel goes up and the other wheel goes down. This causes the bar to twist, and resist body roll. 

Anti-roll bars allow you to "decouple" roll stiffness from pitch stiffness and heave stiffness. This allows you to have a soft suspension in pitch and heave, while being stiff in roll. 

Balance

ARBs are a powerful tool for tuning your car's balance. Imagine if the anti-roll bar was infinitely stiff: body roll would cause the inside wheel to come off the ground. Having a stiffer anti roll bar on one axle will increase the amount of weight transfer on that axle.