For those who need and/or want a much lighter way of handling the graphical user interface than what a desktop environment can provide, a window manager might be the solution. A window manager basically handles the drawing and the placement of the windows
As such, a window manager will not handle tasks like a browsing files, changing the wallpaper, managing tasks… All of the the preceding will have to be added by the user to customize his desktop experience.
A window manager might be used inside a desktop environment, therefore replacing the default manager. For example, Openbox can be used inside KDE instead of Kwin.