OS X's built in screencapture command provides a simple command-line utility to snap images of your desktop. It offers quite a few options and many of them do not quite work as advertised. Even the manual page admits that screencapture is "not very well documented to date".
To make it easier for you to jump in and get started with screencapture, here are six convenient already-tested variations that you can copy, paste and use. Consider adding these capture methods to shell scripts, to system calls from AppleScript or just running them directly from the command line as needed.
1. Capture your primary screen as a (default) png file.
% screencapture ~/Desktop/screencap.png
2. Capture your primary screen as a jpeg (-tjpeg), including the cursor (-C)
% screencapture -C -tjpeg ~/Desktop/mycapture.jpg
3. Wait for five seconds before capturing the screen.
% sleep 5; say "ready"; screencapture ~/Desktop/mycapture.png
4. Interactively (-i) select (-s) an area of your screen with the mouse.
% screencapture -i -s -tjpeg ~/Desktop/mycapture.jpg
5. Interactively (-i) select a window with the mouse. When the crosshairs appear after issuing this command, press the space bar then select a window with the camera.
% screencapture -i -tjpeg ~/Desktop/mywindow.jpg
6. Select an area of your screen and copy it to the clipboard (-c) rather than a file.
% screencapture -i -s -c