Press the Prt Scr key on your keyboard.
The snapshot is saved in memory so you then press paste in an imaging programs such as Paint, Paintshop, Photoshop etc.
you can also press alt+Prt Scr.
This will only take a snapshot of the active window
I use Gadwin Print Screen. It can take screen shots of your entire desktop or shots of just an open application. It's a very versital little program and it's freeware.