Window is the constructor, so it is seen as a function, if you type
window instanceof Window
Will return true, already window is the global variable that is the same as this of the global scope test in the browser console
window === this
You will see that it will return true too, being more specific window is the instance of Window, it is worth mentioning that you cannot do new Window() and yes Window.open, so if you have a declared global variable with a var and in the global scope, if you go to the console and type window.nameVariable will be possible to access it, the same with this.