Why Does Prtsc (Print Screen) Key Not Capture My Screen on My Mac

 Simply hit the Command key (the Apple key next to the space bar), the Shift key, and the number 3 key simultaneously.  This takes a screen shot of the current screen and saves it to the Desktop as a pdf file.

Now, if you only want to copy a segment of the screen, hit Command + Shift + 4, and the mouse cursor will change from an arrow into cross hairs.  Simply click and drag the cross hairs over the area of the screen of which you want to take a picture.  When you release the mouse button, you will hear a clicking sound reminiscent of a camera shutter and a new file will appear on your desktop in pdf format.  Most Macs these days come with a free copy of GraphicConverter, which you can use to convert the resulting pdf file into JPEG, GIF, PNG, etc.

To capture the contents of just one window:

Use cmd + shift + 4, press the space bar and the cursor turns into a camera. Move over any window and you can take a snapshot of JUST that window.

To screen capture to the clipboard instead of a file. If you add the control key to any of the OS X (OS 10.x) screenshot keyboard shortcuts, the screenshot is copied to the clipboard instead of being saved to a file; you can then paste it into any graphics application.