What is the purpose of window manager?

Answer 1

Window managers control the behavior of windows and how they respond to the user. At minimum they at least provide focus control so that an application will know you're talking to it. A window manager is also concerned with how to place or move windows and size them.

A tiling window manager will place windows in a grid and will only allow a user to create more frames and place windows in them, which will result in a highly controlled placement and size for the windows. Windows cannot typically overlap. The disadvantage of this approach being a generally less user-friendly desktop hostile to using the mouse with windows taking up as much real estate as they can without intersecting other Windows, meaning space will be of short supply unless you don't mind some Windows getting much tinier.

A stacking window manager is like what older Windows used to be: You're allowed to overlap windows all you like, size them as you wish, and place them where you want. The disadvantage being you're more likely to rely on the features of the panel (If you have one.) to keep you updated on window activity, as most windows will end up obscured in the end. They also end up relying a little too much on the mouse a lot of times.

A dynamic window manager allows the user to switch between a tiling mode and a stacking mode.

Most modern desktops today, however, use a compositing window manager, which enables advanced window management techniques and enhanced visual effects, it usually takes on many of the disadvantages of stacking window managers (Reliance on the desktop's panel for most events.), solving some others (Thumbnail previews, "live" task switchign where the windows themselves seem to transform themselves on-screen.), and introducing its own, including: A much MUCH heavier memory footprint, requiring somewhat more advanced hardware to work to its fullest. They can be slow if you use more eye candy than window management enhancements. And some compositing window managers on Linux will not work without OpenGL, which can mean trouble with some drivers that doesn't really have a considerable support for OpenGL. Most Linux desktop environments use compositing window managers, but are able to switch compositing on or off as needed, or load alternative window managers if their default choice is incompatible. Also, some compositing window managers and standalone compositors for Linux are quite buggy.

Answer 2

It manages the desktop environment and interface with the user in a graphical format.