Adjusting fore/background FPS in Diablo 3

What exactly is meant by foreground and background FPS? How does it work?

I have adjusted them and tested the different options, but it seems to have no effect on the game itself. I am also wondering what the background really is.

Is it everything that is not my character and other NPCs or is it some sort of background layer?

Max foreground fps is when diablo 3 is your active / focused application you can set this at anything but recommended is 60 or higher. Basically it limits the fps to that number while you are playing the game (setting it at 60 is not the same as vsync though)/

Max background fps is when the game is not focused / active for example when you alt tabbed to a webpage. I recommend setting this between 8 and 20 depending on how good your cpu and gpu are. Basically it is usefull for lowering your cpu usage while the game is alt tabbed / not active