Okay, it's pretty obvious what a mouse is, but for those of you who don't know, a mouse is a device with a ball in it, which you roll around the table, many programs use this to move a pointer around the screen.
If you haven't already got one, then you should go out and buy one of these, for the simple fact that it is highly useful, and many programs allow mouse control, even a lot of the PD stuff. Mouse Control is generally far better than joystick/keys.
The Sam Mouse comes in the form of a white box, which plugs into the mouse port, and a ST compatible mouse, which plugs into this box. If the actual mouse ever dies, then you can simply buy a standard ST mouse to replace it. The actual mouse you get may vary, depending on what stocks have been bought, but all the mice I have seen for it so far were reasonable quality ones.