Press Command, Shift and 3 together at the same time and you'll hear a click sound and you have a Picture #.pdf appear on your desktop. This file is a picture of your entire screen when you took it. If you open the PDF in Preview, you can export it as a jpg, png or gif, or a few other file formats.
There is also Command, Shift and 4, which allows you to select an area to take the picture of, or if you click on a window, it will take the picture of the window.
There is also the application Grab that comes with the system that does the same thing, but allows for timed screenshots.
A positively awesome app for taking screenshots, and movies of what you are doing on screen is Ambrosia Software's Snapz Pro X. Version 2 is coming out soon, and is a complete rewrite and all the beta testers have given it immense praise. However it is shareware and it will cost you some $$ to register it.
Built into MacOS since Mulroney was Prime Minister:
Command-Shift-3 Full Screenshot
Command-Shift-4 Crosshair cursor to select an area for screenshot
MacOSX creates PDF files while MacOS9 and earlier creates editable PICT files.
If you need to make animated movies or select some hard-to-grab items (like menus, which might dissapear if you move the cursor) you can look at 3rd party products like Snaps Pro (MacWorld 4-5 star, from Vancouver BC) or perhaps SnapClip (which I'm unfamiliar with).
New Users: the Command key is the key directly beside the spacebar (both sides); it is marked with a cloverleaf (for Command) and on Apple keyboards, an Apple Logo. Some people call it the "Apple Key", (which is not correct, but is at least descriptive). Get used to calling it the Command key, because that's what you'll find in application docs and help files.
A far more useful screen shot is the window shot, which is also built in.
Press Command-Shift-4 to get the crosshairs and then press the Spacebar. The crosshairs will become a camera and you will be able to take a shot of one window, one icon in a window, the toolbar, the dock, or one item in the dock.