Two Button Mice or not Two Button Mouse
Though Apple has never made a two button mouse, most of the programs in OS X support "right clicking" (that is, clicking the second mouse button). If you have a single button mouse, a right click is achieved by holding the control key and clicking. With a two (or more) button mouse, simply use the right-side mouse button. You'll see different things when you right-click depending on what you right-click on. Accordingly, these are called 'contextual menus', because what you see depends on the context - that is, where you are, what you're doing, and what you clicked on.