Command-Shift-3: Take a screenshot of the screen, and save it as a file on the desktop
Command-Shift-4, then select an area: Take a screenshot of an area and save it as a file on the desktop
Command-Shift-4, then space, then click a window: Take a screenshot of a window and save it as a file on the desktop
Command-Control-Shift-3: Take a screenshot of the screen, and save it to the clipboard
Command-Control-Shift-4, then select an area: Take a screenshot of an area and save it to the clipboard
Command-Control-Shift-4, then space, then click a window: Take a screenshot of a window and save it to the clipboard
In Leopard and later, the following keys can be held down while selecting an area (via Command-Shift-4 or Command-Control-Shift-4):
Space, to lock the size of the selected region and instead move it when the mouse moves
Shift, to resize only one edge of the selected region
Option, to resize the selected region with its center as the anchor point
Formats
Different versions of Mac OS X have different formats for screenshots.
Mac OS X 10.2 (Jaguar): jpg
Mac OS X 10.3 (Panther): pdf
Mac OS X 10.4 (Tiger) and later: png
In Mac OS X 10.4 and later, the default screenshot format can be changed, by opening Terminal (located at /Applications/Utilities/Terminal) and typing in:
defaults write com.apple.screencapture type image_format
killall SystemUIServer
Where image_format is one of jpg, tiff, pdf, png, bmp or pict (among others). If you omit the second line, you will need to log out and in again for the change to take effect.
Grab and Preview
Instead of using the keyboard shortcuts above, screenshots can be taken by using the Grab application included with Mac OS X. It is located at /Applications/Utilities/Grab.
In Mac OS X 10.4, the Preview application can also be used to take screenshots, by using the Grab submenu in the File menu.
From the Terminal
The screencapture command in the Terminal can also be used to capture screenshots, and is useful for scripts. Here is an example.