A trackball is an alternative to a mouse or a stylus. It looks kind of like a mouse upside-down, and you use it by rolling the ball around with your fingers. It has one or more buttons to click, just like a mouse.
Trackballs are great when you don't have enough space for a mouse on your desk-or when you don't have a desk at all, like when you're using a laptop or notebook computer. These days, many little computers, including the Macintosh PowerBook, come with built-in trackballs. Trackballs are also a viable alternative for people who need to use the mouse backwards (with the tail facing them).