If you hold down the Control key and then scroll using your mouse’s scroll wheel, the Mighty Mouse’s scroll ball, or by moving two fingers on the trackpad of your MacBook or MacBook Pro, the entire screen will zoom at the point where the cursor is. If you move your cursor, the screen will move in the opposite direction. To zoom out, just scroll back the other way.
This is a very useful feature for when you want to look at a Web page where some of the text is small (sure, you can press Command-+ [plus key] to increase the text size, then Command-- [minus key] to decrease it, but that changes everything on the page); you might want to get a bigger view of some YouTube videos, or, if you’re a Web designer, you might want to zoom in on something on a page without changing the font to check the finer details.
You can find the control for this on the Mouse tab of the Keyboard & Mouse System Preferences panel. You can disable the feature there, if you don’t plan to use it, or change the modifier key. You can also change how the screen scrolls by clicking the Options button on that preference panel, and checking or unchecking the available options.