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 used to in the mainstream computing world. Some window managers tile, some stack, and some float. Within those three different categories are even more subcategories.
This question in particular consists of all types of window managers, not to be confused with desktop environments. While some window managers can, by design, take the place of a desktop environment, many of the features normally found in a desktop environment will not be included.
Many window managers are used by those who enjoy having complete control over their computing experiences, as well as those who enjoy minimal interfaces. Many often work well on low-end systems because they do not use as many resources to run, due to the majority being minimal by design. Therefore, as they do not use many resources they are often used in distributions that target low-end hardware or power users.
For stacking only window managers, please look here.
For tiling only window managers, please look here.