In case you are a little unsure about what a Window Manager is, here's a brief breakdown.
In (the more recent versions of) Windows you have 3 principle layers:
+ The kernel (controls general hardware calls)
+ The GUI component... MS reckon it's called Explorer. Personally I think it should be called "Lost".
+ The applications.
In Linux you have more layers at work to get your GUI applications runnings.
+ The kernel
+ The X server (on your machine)
+ The X client (on your machine too)
+ The Window Manager
(+ Optionally the Desktop Environment like KDE/Gnome).
+ The application itself.
The X component provides your computer with a way to communicate with graphical hardware and input devices like keyboards and mice. The Window Manager does just that - it controls what your windows, dialogues, widgets, etc all look like. And then the application runs on top.
The application asks the Window Manager to put a window on the screen at position X, Y and size X, Y. The WM tells X how to draw it - what it should look like. X then draws it.
So, there we have it. Flux Box is a Window Manager. It is a very configurable and extremely light-weight WM meaning that it runs smoothly and looks good on almost all hardware.
Hope this hyper-verbosity helps a little