Window Managers are X clients that control the frames around where graphics are drawn (what is inside a window). Linux window managers are plentiful and can be very different from what most users are use to in the main stream computing world. Some window managers tile, some stack and some float. Within those three different categories are even more sub categories.
This question in particular consists of stacking window managers.
Window managers are often used by those that enjoy having complete control over their computing experiences as well as those that enjoy minimal interfaces. Many often work great on low end systems as they do not take as much resources to run, due the majority being minimal by design. Being that they do not use many resources they are often used in distributions that target low end hardware or power users.
For all window managers, please look here.
For tiling only window managers, please look here.