There are two types of control arms: an upper and a lower. They attach the suspension to the chassis, allowing for suspension movement by controlling the motion of the wheels in relation to the vehicle's body. Most modern vehicles just use lower control arms while more high performance vehicles use upper and lower control arms.
The most common failure of the control arms is wear over time. Once the control arms begin to fail, then they will need replacing.
Most manufacturers will cover the control arms under the bumper-to-bumper warranty portion of the factory warranty. There are extended car warranties available that will cover the cost of replacing the control arms, under certain levels of coverage, after the factory warranty expires.