A switch on the mouse -- left, middle, or right. Pressing a mouse button usually initiates some action, which is determined by both the button pressed and the current screen location of the mouse pointer.
The button on top of the mouse. You press it to choose from menus or when you want to move items around on the screen.
Every Windows mouse (or track ball) has at least one button, which when depressed quickly either once or twice initiates some computer action. (See CLICK and DOUBLE-CLICK above.)