How can I turn active desltop off and what is it for? Thanz

Go to the control panel and open folder options. You will see an option to use active desktop or windows classic desktop.
Active desktop was added (as far as I know) to enable you to show updated information from the internet on your desktop, such as weather, stock market info, etc.
If you turn off active desktop, you will be unable to show anything on the desktop of that type. It also will not allow you to use wallpaper other than a few options from Microsoft.