Chances are, even if you haven't heard of Vudu (though that's a little hard at this point), you might just own some piece of content that can be used with the service. Vudu is a digital movie locker that allows users to rent or buy movies online and have them streamed to their computers, or a number of set top boxes and Blu-Ray players with internet connectivity. This is all pretty standard fare. The standout feature, though, is Ultraviolet support, which happens to be one of the preferred methods of offering 'digital copies' for Blu-Ray multipacks.
Once you sign in, you'll have access to your entire library (if you have one), as well as the ability to shop around for anything you'd like to watch.