Ball Joints are part of a cars suspension, they connect the control arms or wishbone to the steering knuckle. A ball joint is used so that some amount of movement and flexibility is allowed while the wheels are steering or moving up and down with the suspension movement.
The shape of a ball joint is more than likely spherical or possibly half round and this is encapsulated or trapped in a shroud or housing. The ball joint has a rod coming out of the housing which normally has a thread on the end or some other way of fixing and this allows the fixing of the ball joint to the steering knuckle. The rod is sometimes tapered which seats in a female taper and then a nut can be used to pull the parts tight together. Sometimes the rod is parallel and can have a groove cut which accomodates a bolt that comes through a part of the knuckle. This bolt can be tightened so that the rod and the ball joint is held captive.
Ball joints can be made sealed with no regreasing being able or they can be fitted with a grease nipple for relubrication and preventative maintenance.
When the ball joint wears it will become loose in it's housing which may cause it to rattle and as a result the suspension may become out of alignment. If this occurs it can also cause excessive tyre wear and noises can sometimes be heard like a clunking when going over potholes.
The wear on a ball joint depends on the mileage of the car and the suspension setup, cars with hard suspension setups and doing high mileages over rough ground can be more susceptible to wear quicker.
Ball joints are checked when the car is taken for an MOT and excessive wear can result in an MOT failure.