[Command][Shift]-3: capture entire screen and save a file to desktop
[Command][Shift]-4: you can choose which area to save.
[Command][Shift][Control]-3 (4): copy the captured to clipboard memory. Then in your 'preview' software (or others like photoshop), choose: File-New From Clipboard.
[Command][Shift]-4, Spacebar: capture a window hovered by your mouse