Bing Desktop is an official Microsoft app that brings the beauty and functionality of Bing straight to your desktop.
The most notable addition that Bing Desktop will bring to your desktop is the wallpaper - it automatically gets that day's Bing image and sets it as your background. Being Bing, this image is always great quality, and, 9 times out of 10, really beautiful. If you click the little information symbol, you'll get more information on both the picture and the photographer.
Bing Desktop also adds a Bing search box to your desktop. Using it is pretty self explanatory, and results will open in your default browser. It can be toggled with the shortcut Windows key + H, or pinned to the desktop, whichever you find easier.
By hitting the little cog symbol on the the Bing Desktop search box, you'll see the app's options. These aren't extensive - you can set Bing Desktop to start automatically, enable or disable the hotkey, and choose whether or not you want Bing Desktop to make the daily homepage into your wallpaper. The only thing we missed was the ability to skip a wallpaper - if you don't like the daily offering, you'll have to set a new one manually - from your own images.
Bing Desktop isn't revolutionary, but it's a quick way for Bing fans to bring the best of the search engine straight to the desktop.