OS X Daily » Ten OS X Command Line Utilities you might not know about

8. screencapture
screencapture offers a more advanced way (over command-shift-3) to take screen captures. To use it, open up your Terminal.app and try typing screencapture -iW ~/Desktop/screen.jpg. It will bring up a camera icon that is waiting to be clicked on a window. Once clicked, a file on your desktop called “screen.jpg” will be created that will contain a snapshot of whatever window you clicked on. You can of course, also take a snapshot of your entire screen by typing screencapture -S ~/Desktop/screen.jpg.