By pressing the Print Screen / SysRq key, The Windows captures the entire screen and copies it to the clipboard. You can also capture only the active window (not the entire screen) by holding down the Alt key and pressing the Print Screen SysRq keys at the same time.
What you will do after? Open a Paint application program or any other graphics editing software like photoshop etc. or even the microsoft word or powerpoint, Then Paste...or simply by pressing the control + V keys and your done.