Take a picture of the whole screen
Take a picture of part of the screen
⌘-Shift-4, then drag to select the area you want in the picture.
To cancel, press Escape.
Take a picture of a window, a menu, the menu bar, or the Dock.
Press ⌘-Shift-4, then press the Space bar. Move the pointer over the area you want so that it's highlighted, then click.
To drag to select the area instead, press the Space bar again. To cancel, press Escape.
Screen shots are saved as files on the desktop. If you want to put the screen shot in the Clipboard, rather than create a file, hold down the Control key when you press the other keys. You can then paste the picture into a document.
You can also take pictures of the screen using the Grab application (in the Utilities folder).