As a user under Microsoft Windows and MacOS, you interact with the computer in a graphic user interface (GUI) -- linux is no different, aside from the wide choice of types
of user interface. Collectively, these different GUIs are known as 'Window Managers' because one important task they perform is controlling the windows that the application programs appear in. As well as what you as a user see there are differences in how these Window Managers work behind the scenes.
Choosing a Window Manager that suits you is just one of the extra choices that linux offers.
History of Windows
The first graphical user interfaces were developed at Xerox Parc in the 1970s. All modern graphical user interfaces user the concepts of 'Window, Icons, Menus, Pointer' developed there. Linux Window Managers work just like other GUIs you may have seen. How they differ is that each one tries in distinct ways to find a better way for you to harness the computers power.
What Window Mangers Provide
Window managers provide a set of functions that are available to programs. For instance that under the KDE Window Manager all of the programs are Internet aware. Under Enlightenment, everything seems to have a colorful graphic appearance and visual impact. Different Window Managers alter the way you can use the computer to a great extent. These differences in 'Look and Feel' can have a big impact on the speed and ease with which tasks can be carried out. They all relate back to fundamental design choices that the programmers made when they set out to write a better GUI. Outside the world of free software you are stuck with a single set of priorities that may not suit you. Under linux, there is intense competition to develop the GUI. You have the choice of several competing GUI paradigms.