The architecture of Mac OS X’s windowing system

You are asking five questions here, and might be better asking five questions ☺ But I'll jump in:

1. X is a server and can be compared to a "web server" in that it is a process that listens for incoming connections that speak a particular protocol (the X protocol) and it issues answers. The connections come from X clients, which may be on the local host or on a remote host accessed over a network.

2. X11 is a "major version" of the X protocol, which has evolved since inception. X11 is the most recent protocol and the most common. (Xorg is an implementation of an X server, X libraries and a collection of clients, all talking X11. There are other implementations: the largely defunct XFree86; commercial ones such as Hummingbird Exceed; forks such as XQuartz, which runs on Mac OS X)

3. A "window manager" is an X client which manages windows. It typically has the responsibility to decorate windows with bevels/borders, drop shadows, a menu bar containing buttons, etc. -- and handle the logic of window placement; support windows being dragged, resized or re-arranged, etc.

4. X requires input and it has a shared responsibility to manage that with the Kernel. Historically, X did a lot of hardware management itself. In modern times, on the Linux platform, X is gradually becoming "smaller" and delegating this responsibility to the Linux kernel. The advantages of this are: smaller X codebase; fewer "crossed-wires" with the kernel and X both trying to manage the same stuff. (an example of this is the Linux event interface, where events from mice etc. are interpreted and translated by the kernel and its drivers into the standard interface at /dev/input/event*, which is consumed by the X server). Note that on other platforms (BSD, Solaris) X is still quite monolithic.

5. There are many, many ways for processes to communicate with each other. I believe X clients can interact by way of the X protocol (for example, traditional X window managers need to know when other clients draw windows, in order to decorate them; pagers need to know when a window's size or position changes, in order to reflect that in the pager). D-Bus is a modern inter-process communication (IPC) technology developed to address the shortcomings of other methods. It is not X specific.

Is a "Display Manager" the same thing as a "Session Manager"?

Answer: No they are not the same. The session managermanages your session, and the display manager is responsible for providing you with a login interface.

Likewise, is a "Windowing system" the same thing as a "Window manager"?

Answer: No they are different. The window mangager sits on top of the Window system.

The Window system: Each currently running application is assigned a usually resizeable and usually rectangular shaped surface of the display to present its graphical user interface to the user; these windows may overlap each other, as opposed to a tiling interface where they are not allowed to overlap.

The window manager: When a window manager is running, some kinds of interaction between the X server and its clients are redirected through the window manager. In particular, whenever an attempt to show a new window is made, this request is redirected to the window manager, which decides the initial position of the window

Session Manager source

In the X Window System, an X session manager is a session management program, a program that can save and restore the current state of a set of running applications.

X window manager source

An X window manager is a window manager which runs on top of the X Window System, a windowing system mainly used on Unix-like systems.

Types of window managers

1. Stacking window managers
2. Tiling window managers
3. Compositing window managers
4. Virtual window managers
5. Window managers that are extensible

The user can choose between various third-party window managers, which differ from one another in several ways, including:

customizability of appearance and functionality:

• textual menus used to start programs and/or change options

• docks and other graphical ways to start programs

• multiple desktops and virtual desktops (desktops larger than the physical monitor size), and pagers1 to switch between them

• consumption of memory and other system resources

• degree of integration with a desktop environment, which provides a more complete interface to the operating system, and provides a range of integrated utilities and applications.

While the main aim of a window manager is to manage the windows, many window managers have additional features such as handling mouse clicks in the root window, presenting panes and other visual elements, handling some keystrokes (e.g., Alt-F4 may close a window), deciding which application to run at start-up, etc.

Display manager source (there is a list of display managers in the source website)

A display manager, or login manager, is typically a graphical user interface that is displayed at the end of the boot process in place of the default shell. There are various implementations of display managers, just as there are various types of window managers and desktop environments. There is usually a certain amount of customization and themeability available with each one.

X display manager source

In the X Window System, an X display manager runs as a program that allows the starting of a session on an X server from the same or another computer.

A display manager presents the user with a login screen which prompts for a username and password. A session starts when the user successfully enters a valid combination of username and password.

The X window system source

Debian manual for x window system

xorg site

The X Window System (X11, X, and sometimes informally X-Windows) is a windowing system for bitmap displays, common on UNIX-like computer operating systems.

X provides the basic framework for a GUI environment: drawing and moving windows on the display device and interacting with a mouse and keyboard. X does not mandate the user interface — this is handled by individual programs. As such, the visual styling of X-based environments varies greatly; different programs may present radically different interfaces.