Solution:
To take a snapshot and save it to a file that will appear on your Mac's desktop...
Cmd+Shift+3...........................................Entire screen
Cmd+Shift+4...........................................Specified region of the screen
Cmd+Shift+4 and then space bar...................Specified window
To take a snapshot and place it in the copy buffer (use Cmd+v to paste it into an app)
Cmd+Ctrl+Shift+3...................................Entire screen
Cmd+Ctrl+Shift+4...................................Specified region of the screen
Cmd+Ctrl+Shift+4 and then space bar...........Specified window