On some Laptop Keyboard, there is a small rubber or foam button in the middle of the keyboard that can be used to control the mouse. On the one I'm using it's a circular rubber one below the letters G and H and above the B.
I don't really understand why the button is there - I just always use the touchpad and can't really envisage why you would use this keyboard button. Is it just an alternative way of mouse moving for those who don't like touchpads, or is there something more sophisticated that it offers?