a type of
tripod head. It is composed of two parts, namely a
socket that
usually remains static and fixed to the
legset and a
ball part that is attached (usually via a
quick release system) to the
camera or to the
lens.
Normally you try for balance, so if you have a heavy lens like a very big telephoto lens you attach the head to the lens itself.
When you use a ballhead, basically the ball swivels in the socket and provides a support point for the camera. Ballheads can be locked in a certain position, usually by means of a rapid locking brake system. Some ballheads can also be tuned to a certain amount of friction, conditioned by lens weight and photographer preference.
Ballheads are suitable for nature photography, when the subject moves rapidly and you want to track it.
Ballheads can be somewhat delicate: you don't want to bang them around or to let dust and sand get between the ball and the socket.