For the latter, once you use the shortcut, your mouse pointer will turn into crosshairs, and you would click on one point of the screen, then drag and release your mouse to take a capture of the selected area. Pressing Command-Shift-3 will take a screenshot of the entire screen, while Command-Shift-4 will let you take a screenshot of just a selected area of the screen instead. This video demonstrates how easy it is to take a screenshot with shortcuts in Mac OS X.
