There are numerous reasons why you might want to take screenshots of what is happening on your desktop. If you are creating a tutorial guiding people through a particular task, screenshots are perfect for illustrating things in a way that everyone can understand. If you have a blog and you decide to review a web site or a game, you’ll probably want to include screenshots.
There’s nothing to stop you from simply hitting the Print Screen key on your keyboard and pasting the image from the clipboard into your favorite image editor, and if you have Windows 7, you could use the Snipping Tool to gain access to a few more options. But if you want complete control over the screen capture process, you might be interested in taking a look at Screenshot Captor.