From the GConf key description:
The window focus mode indicates how windows are activated. It has three possible values; "click" means windows must be clicked in order to focus them, "sloppy" means windows are focused when the mouse enters the window, and "mouse" means windows are focused when the mouse enters the window and unfocused when the mouse leaves the window.
When I configure focus_mode using gconf-editor, /apps/metacity/general/focus_mode, I can't tell the difference between mouse and sloppy.
When I set this key as mouse, the focus is still on that window if I hover the cursor to the background.
Best Answer
The mouse mode will set focus on the desktop when the mouse hovers over a desktop area that has no open window, whereas the sloppy mode does not. The sloppy mode changes focus only if the mouse hovers over a window.