In a standard wishbone suspension there are two wishbone shaped metal arms (sometimes called A-Arms) per wheel. Each wishbone is mounted to the chassis at two places using bushes or ball joints and to the knuckle in one place. The coil spring and damper is attached to the rear wishbone of the pair.
Their role is to precisely control the motion of the wheel during travel. Although they are sometimes called A-Arms, they can in fact also be L shaped or H shaped, with L shaped brackets preferred for passenger vehicles as the drive can be more comfortable and have good handling.
The wishbones are configured slightly differently to front wishbones. Where two wishbones are used on each wheel, in rear suspension you can have a pair of wishbones on each side of the wheel, meaning a total of four wishbones per wheel.
In the failure of a wishbone you will notice a very loud rattle, vehicle vibration and lack of control over wheel positioning. Damage is more likely on ball joints where wishbones connect to the vehicle and can be caused by corrosion.