Back in the days of DOS, the PrintScrn key used to print the
screen. But in all versions of Windows, this works differently,
and the name of the key is now an anachronism.

To use the key, press it to capture an image of the entire
screen, or press alt-PrintScrn to capture an image of the active
window. Either one captures the image to the Windows clipboard.
Once it's in the clipboard you can paste (Ctrl-V) it into any
application that supports graphics (Windows Paint, other graphics
programs, even your favorite word processor). You can edit or add
to the image as you wish, then E-mail to your friend or print it. 

I find it's best to have the graphics program open already. somehow the
clipboard contents get emptied if you PrtScn then open the app.

PrtScn copies full desktop to Windows Clipboard
Alt-PrtScn copies active window to Windows Clipboard

You will see nothing visible happening when you do this.  Paste from
Clipboard (Menu: Edit/Paste or Ctrl-V keys) into an application that can
accept an image, e.g. Word, Wordpad, Paint, etc. to see what your screen
shot.  Run the clipboard viewing program clpbrd.exe to see the clipboard
contents.  More help on this available in Windows Help.