The Windows Print Screen Key can be a very powerful tool if you know how to use it. Often abbreviated Prt Scr, the Print Screen key is a supported on most PCs.
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When you push the Print Screen key it copies a bitmap image of the screen - a picture of your screen, often called a "Screen Shot" - onto your Windows clipboard.  
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The clipboard is a holding place on your computer where you can store ONLY the last thing you cut or copied. Whatever is on your clipboard stays there until you either replace it with something else, or shut down the program you're using.  This means you can paste the item in your clipboard over and over again if you need to.

Let's try it now...

Press the Print Screen key.  Next you'll need to open any program that you can paste a bitmap into.  
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We'll open WordPad... go to START, Programs >, Accessories, WordPad and then press Control + v to paste the image - or go to Edit, Paste. Ta-daa! There's a picture of your screen!

Alt + Print Screen...

If you have several different programs open and only want a Screen Shot of one of them, click into that program to select it. Next, hold down the Alt key on your keyboard, and press the Print Screen key (Alt + Print Screen). Continue with the Paste process described above.

What now?

You can save the file containing your Screen Shot and send it via email - or print it out to use it in a presentation, or create a handbook of screenshots showing how to perform a particular series of steps in a program.  
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If you have a graphic editing program on your computer, you can paste the Screen Shot into it, edit it any way you choose, and save it as a gif or jpeg.  Once saved the image can be inserted  directly into an email, or any other program.
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How about those cryptic error messages that pop up when something in a program goes haywire?  How do you remember exactly what it said when you call or email tech support?  Hit your Print Screen button and take a picture of it so you'll know exactly what it said!
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Want to show someone your company's website even though you don't have access to a computer?  Take a Screen Shot of each page, paste them into WordPad - or any other program - and print them out for a presentation you can show to clients! 
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The possibilities are endless.  Once you get the hang of it, you'll be using the Print Screen key all the time!