The Mac has several options for capturing screen contents as follows:

COPY TO A DESKTOP FILE
To capture a window, press Shift-Command-4 and then the space bar. Click the window to capture. For a selected area, press Shift-Command-4 without the space bar. Drag to frame the area and let go.

The default graphics format is either a PDF or PNG file, named Picture 1, Picture 2, etc., but the file type can be changed with utilities such as TinkerTool.

To capture the entire screen, press Shift-Command-3.


COPY TO THE CLIPBOARD
To capture a full window, press the Control key along with all the other keys above. For example, to capture a window, press Ctrl-Shift-Command 4 and then the space bar. Click the window to capture.