Ball Joint

On modern vehicles, joints are the pivot between the wheels and the suspension of an automobile. They are today almost universally used in the front suspension, having replaced the kingpin/linkpin or kingpin/trunnion arrangement, but can also be found in the rear suspension of a few higher-performance autos. Ball joints play a critical role in the safe operation of an automobile's Steering And Suspension.

Ball joints are a part of the suspension system. Basically, ball joints provide a pivot point for each front wheel to pivot, or turn upon, as the steering system turns the front wheels left and right and the control arms move up and down.
Each of the control arms of your front suspension system, upper and lower, have a Ball Joint at the outermost end of the control at its closest point to the front wheel. In the case of strut-type suspension, there is only one Lower Ball Joint affixed to the Lower Control Arm at the same point.
This ball and socket type configuration creates the ability for the wheels to turn left and right and move vertically with changes in the road surface while at the same time supporting the weight of the vehicle and providing a comfortable ride.