What is X Windows?

X Window System is a networking and display protocol which provides windowing on bitmap displays. It provides the standard toolkit and protocol to build graphical user interfaces (GUIs) on *NIX operating systems and OpenVMS, and is supported by almost all other modern operating systems.
Desktop Managers for Linux
Part of what makes Linux useful on your computer is its graphical user interface. The GUI gives Linux a “look and feel” with clickable icons and widgets, as well as screen borders, scroll bars, and menus that the user can manipulate and customize.
This “point and click” environment makes the operating system more intuitive by presenting interface options in an attractive visual layout that doesn’t require knowledge of textual commands. Without the GUI, Linux (or any operating system) requires users to type commands in a procedure that is known as the Command Line Interface (CLI).
What is a Desktop Manager?
In graphical computing, a desktop environment (DE, sometimes desktop manager) offers a graphical user interface (GUI) to the computer. The name is derived from the desktop metaphor used by most of these interfaces, as opposed to the earlier, textual command line interfaces (CLI). A DE typically provides icons, windows, toolbars, folders, wallpapers, and abilities like drag and drop. As a whole, the particularities of design and function of a desktop environment endow it with a distinctive look and feel.
Elements of Desktop Environment
A desktop environment is made of several parts, such as :
1. Window Manager

2. File Manager

3. Application programs(optional)

1. Window Manager
The most fundamental part of a DE is the window manager or WM. A window manager creates a certain way for application windows to present themselves to the user. It manages the various application windows, keeping track of which ones are open and providing features to switch between them.
2. File Manager
This application manages files/ folders and presents them in a way that the user finds convenient. It provides some basic file operations like viewing, copying/ moving to different locations and deleting. DEs usually provide a way to set wallpapers and screensavers, display icons on the desktop, and perform some administrative tasks.
3. Application Programs
Desktop Environments may optionally include application programs such as word processors, CD/DVD writing applications, web browsers and e-mail clients. Some exceptions must be noted here. Window managers like Fluxbox, wmii and Ratpoison are capable of operating independently of a desktop environment and were written with this objective in mind.
Additional hand-picked applications add functionality and hence render them capable of operating as a full-blown DE. For this reason, they are considered to be lightweight desktop environments. This contrasts the behaviour of WMs like Metacity and KWin which were not written with the objective of operating independently of a DE.