A steering knuckle is an aptly named part that is basically a ball in a socket that can rotate in all directions and transmit force. They are used in almost all vehicles in the steering linkage to transfer lateral motion (side to side movement) into rotational movement (rotation of the wheel about the ball joints).
The Fiero uses two of these guys in the rear to set the toe (tracking) of the rear wheels. It doesn't really rotate in the sense of the fronts, but does allow movement in the vertical direction as the wheel moves up and down.
From: Tin Man