Many operating systems enable you to take a basic grab of your desktop.
Within Windows, you can press the 'Print Screen' button on your keyboard and
that should take an instant grab of your entire screen.
However, this doesn't always work and isn't always the best way to grab your
desktop. Sometimes you might find that some applications (widgets, perhaps)
don't show when you use this function.
If you want to grab a particular section of your desktop, such as a
particular application, grabbing the entire screen is overkill and requires a
lot of system resources if you've got a large monitor.
Screenshot Captor is a free tool that enables you to take a grab of your
entire screen, a specific window or simply a section of the screen. It's a
superb little tool that competes with the commercial screen-grabbing tools on
the market.
You can choose to include the cursor within your grab, the amount of
compression you want to apply to your image, add special effects to the grab
and, once you're happy with the results, export to a number of image formats.
Note that this application requires a little configuration before use, for
instance turning off the automatic image compression before grabbing your
screen.