Wubi is a way to install Ubuntu Linux on your computer without having to take the risk of creating a dedicated partition. This is great for someone who wants to try out Linux. What Wubi does is it installs Ubuntu as a Windows program, so it can be installed and uninstalled just as easily as any other Windows Program.
I've wanted a dual boot setup on my laptop for a while now as I use both Windows and Linux for various tasks. Last time I attempted to partition a hard drive to dual boot my desktop computer, I ended up having to reformat the drive to get it to work again. Fortunately I'd backed up my data. That computer now only runs Linux, but for my laptop I wanted a dual boot, but didn't want to risk destroying Windows this time. This is where Wubi comes in. I was wary of Wubi at first, and didn't actually try it until it was officially supported by Ubuntu. The installation was easy, and it even let me choose what desktop environment I wanted. It took awhile for it to install as it needed to download Ubuntu first. The installation is finished when the computer is rebooted and Ubuntu is choosen from the boot menu. The rest of the installation was also very straightforward. There were no installation issues, no partitioning, and Windows still worked!
Now when the computer starts up, I'm given the choice of booting into Windows or Ubuntu. If I don't respond after 15 seconds, it automatically boots into Windows. The Wubi installation of Ubuntu works just as well as Ubuntu on a dedicated partition. I haven't had any issues with the OS running any differently and it runs just as fast. Updates are also installed regularly just as a dedicated Ubuntu partition would do without having to reinstall Wubi.
Currently, the Wubi installation of Ubuntu is incapable of hibernating. This doesn't bother me as I don't typically use hibernation anyways. What can be a problem, especially for someone who installs Wubi on a desktop computer, is Wubi can't be hard-rebooted (forced restart or power failure) or it could potentially damage both Ubuntu and Windows (fortunately, it can be easily fixed).