A sway bar is a thick metal rod that spans the width of your vehicle from shock absorber to shock absorber that helps your vehicle counteract the "roll" it experiences when entering a turn. It's a part that rarely fails on its own unless there was a manufacturing defect. Usually, it's the bushings or the links that connect it at the ends that wear out and need to be replaced. If you do need to replace the entire sway bar, most likely the vehicle has either been in an accident or you're restoring a vehicle and the original has lots of rust and corrosion.
A sway bar is usually about ¾-inch to about 1 ½-inch in diameter and it can be several feet long. Most vehicles have one in the front and one in the rear and they span from the shock absorber on one side to the shock absorber on the other side. When you enter a turn, the centrifugal force wants to make the car roll or lean to the outside of the turn. The downside to this effect is it reduces the traction on the inside tires and shifts more of the weight of the vehicle to the outside which can make a rollover more likely. With the sway bars installed, the downward force on the outside wheels is counteracted by the bar "pushing back" from the other side of the car. This helps keep traction on the inside tires and reduces the chance of a rollover by keeping the car more level with the ground.
If your vehicle needs to have one or both of its sway bars replaced, look no further. Car Parts Discount has a selection of sway bars and sway bar parts available for a wide variety of vehicles at the lowest possible prices.