Really it's differ, but actually has the same function in the end. You now that alt-tab key will bring a small window that allowing you to choose or switch to another application, if there more than 1 application are running.
Since the alt-tab will switch over window, what happened with Winkey-Tab? It is simply switching over window, but the effects wouldn't the same as alt-tab does. It's affecting the Taskbar, and it will choose the application window which still remain on Taskbar. Hardly understand what i'm trying to say? The best thing is "Just Try It".
Don't forget to open several Window(s).