a "screenshot" (also called "screenie" and "screendump") is an image of your monitor.
To make one of these, simply press the "PrtScn" button on your keyboard (hold "Alt" while pressing "PrtScn" to take a picture of the active window only)
Next open your photo-editing software and create a new image. Paste the ScreenDump (as it is saved in your clipboard) into the new image.
Now simply save the image