What does "Win Button" + "Print Screen" do on Windows 8?

Solution 1:

It takes and saves the screenshot. It places the saved screenshot into your pictures folder as a PNG.

For reference: Windows 8: Capture Screenshots (en-US)

Solution 2:

As Ben and user142485 point out, Win + Print Screen takes a screenshot, dims the screen to let you know something happened, and saves a .png file to your pictures. This is an enhanced version of the screenshot functionality that has been in Windows going back to at least Windows 95. It might have even been in Microsoft Bob or Me, but thank goodness I never had the chance to find out. Good-bye Paint, or Gimp, or whatever else you used to paste the screen shot into before saving.

Curiously, Alt + Print Screen, which has also been around for just as long, continues to behave the way it did in previous versions of Windows. It captures a screen shot of the active window. However, the screen doesn't dim, and the screen shot is NOT saved to your pictures, so you'll need to continue to use Paint in this scenario.

Solution 3:

To add to the other answer; I suppose it lowers the brightness to let you know a screen shot just occurred, just like on an iPhone, or when a photo is captured from Photo Booth on a MacBook.