This stands for "Print Screen," and pressing this button saves an image of your screen to your clipboard. This is similar to clicking "copy" on a picture.  The button is usually in the upper-righthand corner of your keyboard, above "Backspace." Press "Prt Sc" once to take a picture of your current screen. If you hold the "Alt" key while doing this it will take a picture of just the window you are using, such as your internet browser. So if you want a shot of only your internet browser, click into that window and press Alt & Prt Sc keys together. (Microsoft office word may also work) This free program comes pre-installed on every Windows computer, and it allows you to paste your screenshot and edit it if you'd like.  You can find paint through the Start Menu. Click "All Programs" → "Accessories" → "Paint" to open it. You may use any program that allows you to paste images -- Photoshop, Microsoft Word, InDesign -- but Paint is the simplest and fastest way to save a screenshot. The paste button is in the upper left corner of Microsoft Paint, but you can also press the CTRL and V keys simultaneously to paste. You can now save your screenshot through Paint. Click the small purple save button (it looks like a purple floppy disc) or press CTRL and S simultaneously. You will be given a chance to name the picture and choose the image quality. Microsoft put a small tool on every windows machine that lets you take custom screenshots. In the start menu, search for "Snipping Tool" to find the program. From here, you can create a custom sized screenshot and save it directly through the Snipping Tool program:  Click "New" Click and drag over the area you want a picture of. Click "Save Snip" (square, purple floppy disc button).

Summary:
Find a key on your keyboard labeled "Prt Sc". Open Microsoft Paint. Click "Paste" to view your screenshot. Save your screenshot. Alternatively, use the Snipping Tool on Windows Vista, 7, or 8.