You use trackballs to input relative movement commands. The position of the ball doesn't need to relate to the position of the cursor. It's like a mouse if you

You use trackballs to input relative movement commands. The position of the ball doesn't need to relate to the position of the cursor. It's like a mouse if you picked it up and replaced it after every stroke. They have a couple nice properties in that they can be fixed in place and offer more freedom of movement to a joystick.