There are two types of control arms, an upper and a lower. Most modern vehicles just use lower, while more high performance vehicles used both. The control arms are part of the vehicle's suspension system. They attach the suspension to the chassis, precisely controlling the wheel movement. The control arms tend to wear over time. Once they are worn, a control arm replacement is necessary.
When performing a control arm replacement, the technician removes the wheel and detaches the control arm from the chassis and suspension. Then the worn control arm is removed, along with the bushings. Finally the new control arm and bushings are installed and the suspension is reassembled.
Most manufacturer will cover a control arm replacement under the bumper-to-bumper portion of the factory warranty. After the factory warranty expires, there are extended auto warranties available that will cover the cost of a control arm replacement.