In Mac OS X Leopard, you can capture an image of your entire screen by pressing Command-Shift-3.
With Command-Shift-4 lets you choose a specific part of your screen to save as a screenshot: Click and hold to place the small cross-hair cursor at one edge of the area you want to capture, then drag horizontally and/or vertically to select. When you release the cursor, the screenshot is saved to your desktop.
But creating screenshots this way often means you need to crop or clean up the edges of the image later. That’s especially true if you’re planning to use it as a graphic element in a document or presentation. Fortunately, Mac OS X Leopard offers a way to save clean screenshots of individual elements on your desktop — such as Finder windows, menus, icons, or the visible portion of an open document — without capturing anything else in the background.
Hold down the Command, Shift, and 4 keys, then press the Spacebar. Instead of a cross-hair cursor, a small camera icon appears. When you move this camera icon over the element you’d like to capture, that element is highlighted. Click your mouse or trackpad, and you’ve captured a screenshot of just that element — no further cleanup required.