OS X Screen Shot Techniques.
Method: Command-Shift-3 takes a screen shot of your entire
screen. The file is saved on your desktop as a PDF document titled
something like "Picture 1."
changes your cursor to a small crosshair icon. Only the part of
the screen you draw a box around with that icon will be saved in
your PDF screen shot.
gets you the crosshair icon again, but this time after you see the
icon, tap your spacebar once. Your icon should change to a small
camera. Now if you click on a window or dialog box, Mac OS X automatically
saves a cropped screen shot of just that window.
Clipboard Trick: Add the Control key to any of the techniques
listed above (i.e. Control-Command-Shift-4), and your screen shot
is copied directly to the clipboard. So if you need to paste a screen
shot into Photoshop (like I've been doing here) this extra key lets
you skip the PDF file altogether.