Shift-Command-3 captures your entire screen and saves the screenshot on your Desktop.

Shift-Command-4 changes your mouse pointer to a crosshair (+), after which you click and drag (from upper left corner to bottom right corner of area) to select an area of your screen to capture. Screenshot is saved on your Desktop.

(Note: Command key = Apple key, next to space bar)

Default format is TIFF.

To change format to JPEG (or PNG or PICT), open a Terminal window, type in the following line and hit Enter:

defaults write NSGlobalDomain AppleScreenShotFormat JPEG

For PNG or PICT (or back to TIFF) format, just substitute for JPEG in the above command.