Ball joints are types of mechanical joints often found in vehicles and machinery to assist with movement of parts and components. Ball joints consist of a spherical head (usually a metal or plastic stud) with a thread and a spherical housing cavity. The stud rotates within the housing to allow for a limited range of movement in all directions rather than just left or right.
Like the ball and socket design that allows for rotational movement of your hip joint, ball joints allow for a degree of rotational movement between two equipment parts. Single ball joints are used to allow free movement in two planes simultaneously.
They're useful in connections where you desire a degree of rotation in movement. A similar type of joint, the rod end, has a threaded shaft attached to a circular head with an opening.
Ball joints are an important suspension element found in vehicles such as cars, where they will usually be combined with another ball joint with control arms allowing movement in three planes.
Whilst a common element used in steering wheels and suspension, they are also used in numerous other applications from robotics and toys, to other apparatus requiring mechanical linkages such as aircraft components and solar panels.