If it's the one I'm familiar with, it just places a Windows background "plate' behind it. There's supposed to be a GIMP build out there that is more like Photoshop. Most things have been renamed to be consistent with PS naming conventions, although I can't recall if it actually transmogrifies the UI to look like PS.
My User Contributions master list - my initial post links are down but scroll down page to find list to active links
