The correct term for this is a "screen shot".  There are a couple of keyboard shortcuts that will allow you to do this, but you will also need some type of graphics editing program, paint, ms word, or something similiar.

The keyboard commands are ALT + Print Screen,  CTRL + Print Screen.  One gets just the active window, the other gets everything on the visible screen.

Once you press one of these keyboard commands, the image will be in your clipboard.  You need to then open up image processing software (paint, photoshop, etc),  and PASTE what is in memory onto a blank document.

That's all there is to it!