A control arm is a bar with a pivot at each end, used to attach suspension member to the chassis.
When coil springs are used in both front and rear suspension, three or four control arms are placed between the rear axle housing and the frame to carry driving and brake torque. The lower control arms pivot on the frame members and sometimes support the rear coil springs to provide for up-and-down movement of the axle and wheel assembly.
A-arms are control arms with two inboard pivots, giving strength.
Some front end designs use control arms instead of A-arms. usually to save weight and add adjustability.