A ball head is a tripod head where the camera movement is locked with a single lever. The camera is mounted on a post fixed to the ball and when the head is slackened off the ball can move freely in any direction.
Some ball heads also have a friction control so that the camera doesn’t just flop loosely as soon as the ball is slackened, and still needs a little effort to change its position.
Many ball heads also have separate pan movements, so that you can turn the camera horizontally without having to slacken off the ball.
Ball heads are more compact than three way heads and popular for landscape and other outdoor photography. They don’t offer the precise camera positioning of three way heads, but they are fast and simple to use.