The original, held-over-from-Classic method for screen capture is the venerable keyboard shortcut (sometimes referred to as an 'F-key'), Command-Shift-3. Hitting this key combo creates a 'Picture 1.jpg' (or .png, .pdf or .tiff file, depending on your OS version and whether you've fiddled with your settings via Terminal or AppleScript utility) that includes your entire desktop, multiple displays and all. Too much of a good thing? Try Command-Shift-4, and drag the crosshairs over the target area (hit Spacebar to toggle a 'camera' icon that will capture a full window with a click; hit Esc to get out of capture mode). As you use these keys, and a collection of 'Picture N.jpg' files begins to accumulate on your desktop, try adding the Control key to the combo -- Ctrl-Cmd-Shift-3/4 sends the captured image to the Clipboard, rather than a file. Personally, I use the clipboard version of the 4 key all the time to copy snippets of my iPhoto images into outgoing mail. It's faster & easier than exporting the full-size image.
Sometimes, unfortunately, a command key is not quite good enough; for example, it's tricky to capture a pull-down menu onscreen. For this, there is Apple's Grab application, hiding out in your Utilities folder (also available as a Service in many applications). In addition to basic capture acrobatics, Grab offers a Timed Capture tool that works exactly like the timer button on a camera: hit the Capture menu/Timed Screen (or Command-Shift-Z), start the timer, get your ducks in a row and wait ten seconds. You can even Cmd-Shift-4 the resulting full-screen image to snag a small section if that's what you need. Happy snapping!