Second monitors that are connected to a machine with already one or more monitors installed are often set to serve as extensions of your desktop by default. In some cases, e.g. if your second monitor is a television and you want to watch a movie running on your computer, it might be helpful if your second monitor duplicates what's running on your first.
To accomplish this is really easy on Windows 7 and 8: Just right-click your desktop and select Screen resolution. If you connected both screens to your machine already, these are the options you'll see:
Under Multiple displays, select Duplicate these displays from the dropdown menu and click OK afterwards