Description: A ball joint consists of a hardened-steel, bearing stud and socket enclosed in a steel housing. The bearing stud is tapered and threaded and fits into a tapered hole in the steering knuckle. A protective boot prevents dirt from entering the joint assembly. Ball joints are used on the front end of virtually every car and light truck. Vehicles with conventional suspension systems use two ball joints per wheel: an upper and a lower ball joint. Cars with MacPherson strut suspension systems use a lower ball joint and an upper strut bearing rather than an upper ball joint.
Purpose: Ball joints serve as the pivot points between the tires and suspension. Ball joints also support weight. On some vehicles, the ball joints may be used to make wheel alignment settings.
Maintenance Tips/Suggestions: Many cars use “lubed for life” ball joints that can’t be lubricated. Replacement ball joints often come with lubrication fittings. If your car has lubrication fittings, the ball joints should be lubricated at every oil change. Ball joints, along with other suspension components, should be inspected annually along with a complete wheel alignment. The most common vehicle symptoms associated with worn ball joints are wandering, uneven tire wear, and erratic steering. Ball joint inspection methods and specifications vary, so have your vehicle inspected by a qualified service professional.
source : carcare.org