If you're not familiar with it, (you may have just typoed), DE refers to a desktop environment, xfce being one of the lighter ones, there are many others like Gnome, KDE, and so on. WM refers to window managers, like dwm and openbox. The difference is that DE's often have their own tools to set up printing, networking, etc, whereas with a WM, you usually have to set up such things yourself. It's a matter of preference, many of us like the added flexibility and lightness of a WM when compared to a DE.