THe print screen key takes a picture of what is currently displayed on the screen. To get the image, open a photo editor, such as paint and press Ctrl+V to paste the image on the screen.
I dont know how the SYS RG key does. Im sure its system request but I have never come across how it works or its functions.
The pause break key acts as a fast way to pause a game. I dont think it has any main function around the desktop area of the computer.
Hope this clears things up.