Click on your desired window to take a screenshot of it. Press and release the space bar: The crosshair turns into a little camera icon, which you can move over any open window. You have a number of other options after hitting Shift-Command-4: Release the mouse button or trackpad to take the shot. Use this keyboard combo to turn your cursor into a crosshair, which you can drag to select a portion of your screen to capture. This keyboard shortcut captures a screenshot of your entire screen.
