Just to sum this all up and add some detail: Shift-Cmd-3 will simultaneously capture a screenshot of ALL monitors connected to your system and put the files on

Just to sum this all up and add some detail: Shift-Cmd-3 will simultaneously capture a screenshot of ALL monitors connected to your system and put the files on the Desktop. Shift-Cmd-4 will give you a cursor with which you can draw a rectangle of the area desired; if you then hit Space, however, the cursor changes to a camera icon which then allows you to click on any open window. That window will then be captured. Shift-Cmd-5 pulls up a full set of screen capture controls with various options.