There are various ways in which to take a screenshot on your Apple MAC;
Take a screenshot of the entire screen
Command-Shift-3: Take a screenshot and then save it as a file on your desktop
Command-Control-Shift-3: Take a screenshot and save it to the clipboard
Take a screenshot of a select area
Command-Shift-4, then select an area: this saves it to your desktop
Command-Control-Shift-4, then select an area: this saves it to the clipboard
Take a screenshot of a window
Command-Shift-4, then space, then click a window: this saves it to your desktop
Command-Control-Shift-4, then space, then click a window: this saves it to the clipboard