A MOO , an acronym for (MUD object oriented), is a text-based online virtual reality system to which multiple users (players) are connected at the same time,
The term MOO is used in two distinct, but related, senses. One is to refer to those programs descended from the original MOO server, and the other is to refer to any MUD that uses object oriented techniques to organize its database of objects, particularly if it does so in a similar fashion to the original MOO or its derivatives. Most of this article refers to the original MOO and its direct descendants, but see Non-Descendant MOOs for a list of MOO-like systems.
The original MOO server was authored by Stephen White, based on his experience from creating the programmable TinyMUCK system. There was additional later development and maintenance from LambdaMOO administrator, and former Xerox PARC employee, Pavel Curtis.
One of the most distinguishing features of a MOO is that its users can perform object oriented programming within the server, ultimately expanding and changing how the server behaves to everyone. Examples of such changes include authoring new rooms and objects, creating new generic objects for others to use, and changing the way the MOO interface operates. The programming language used for extension is the MOO programming language, and many MOOs feature convenient libraries of verbs that can be used by programmers in their coding known as Utilities. The MOO programming language is a domain-specific programming language.

Background
MOO, along with all of its nephews, started out with text based adventure games. With the advent of the internet, MUD was formed as a networked version of one of those games. Eventually it developed into a tree of different types of MUD, with MOO becoming one of them.
Stephen White (also known by the handles "Ghondahrl" and "ghond") wrote the first version of the MOO server, which was released on May 2, 1990, and used for the operation of a server called "AlphaMOO". Pavel Curtis, an employee of Xerox PARC and also known by his handles "Lambda", and "Haakon", took the basic design, language, and code, fixed bugs and added features to release the first version, called "LambdaMOO" on October 30th, 1990.
According to Jill Serpentelli in her paper Conversational Structure and Personality Correlates of Electronic Communication:
Curtis went on to explain how the transition occurred from AlphaMOO to LambdaMOO. After fixing bugs in the system, rewriting some of the code, adding more programming capability, and writing documentation, he had created what he termed "a truly separate entity" from the original AlphaMOO. He dubbed this new system LambdaMOO, after one of his names on the system and, according to Curtis, "because it's a key word in some of the other non-mud research that I do." The new system was announced as open for public access on UseNet (a world-wide bulletin board system) in February of 1991 (Curtis, personal communication).
MOO was originally developed as a MUD server in the same general style (sharing much of the command syntax and community conventions) as TinyMUD.
There are currently two distributions of the MOO server code. The more popular of the two, the LambdaMOO server, is named such as indication of the close historical and continuing association of the MOO server code with the first public MOO, LambdaMOO, which is still popular today.
It is this LambdaMOO version of MOO that gained popularity in the early 90s, and it remains the most widely-used distribution of MOO. Pavel Curtis continued to maintain the server for several years. Other early contributors to the LambdaMOO server included users Tim Allen ("Gemba"), "Gary_Severn", Roger Crew ("Rog"), Judy Anderson ("yduJ"), and Erik Ostrom (known as "Joe Feedback"). Later, Erik Ostrom maintained the server, and the server is now maintained by Ben Jackson and Jay Carlson and has a LambdaMOO SourceForge.net project.
Sometime around early 2005, the GammaMOO server forked from LambdaMOO with the goal of being a testing ground for new features not yet qualified for inclusion in the main MOO distribution (which has very strict standards for any changes). It can be seen as a the equivalent of a "development branch" that most other projects have.

History
Some servers use MOO style object oriented characteristics without being descended from the original MOO server, in the sense that they use little or none of that server's source code and use internal languages that are more or less incompatible with the MOO programming language. None of them have attained the popularity of LambdaMOO or its relatives.
Stephen White went on to write a new and similar system called CoolMUD, although it never obtained the same wide userbase as MOO. Another, later, attempt at a programmable object-oriented MU* server was ColdMUD, written by Greg Hudson and later maintained by Brandon Gillespie under the name "Genesis".
One unusual MOO with no real relationship to the original MOO is called mooix. mooix is unique among MUDs in that it uses the underlying UNIX operating system to handle all of the multitasking and networking issues. Several unique side effects result from this, one of which is that the MOO can be programmed in any language. mooix was written after a failed attempt by Joey Hess to write a MOO entirely in Perl, called perlmoo.
There are a number of MOOs written in Python, including POO, MOOP, and playsh.

Non-Descendant MOOs
Participants (usually referred to as users) connect to a MOO using telnet or some other, more specialized, client program. Upon connection, they are usually presented with a welcome message explaining how to either create a new character or connect to an existing one.
Having connected to a character, users then give one-line commands that are parsed and interpreted by the MOO as appropriate. Such commands may cause changes in the virtual reality, such as the location of a character, or may simply report on the current state of that reality, such as the appearance of some object.
The job of interpreting those commands is shared between the two major components in the MOO system: the server and the database. The server is a program, written in a standard programming language, that manages the network connections, maintains queues of commands and other tasks to be executed, controls all access to the database, and executes other programs written in the MOO programming language. The database contains representations of all the objects in the virtual reality, including the MOO programs that the server executes to give those objects their specific behaviours.
Almost every command is parsed by the server into a call on a MOO procedure, or verb, which actually does the work. Thus, programming in the MOO programming language is a central part of making non-trivial extensions to the database and hence the virtual reality.

MOO access
All MOOs provide a flag called Wizard; when set on a player, the player gains the ability to view and modify nearly everything in the MOOs database. Such players are called Wizards, and usually form the basis for MOO administration.
Wizards are able to restrict access to the MOO, as well as make news postings and monitor logs. Wizard permissions are needed for modification and even execution of verbs and properties for which the user does not own, or is not publicly readable/writable. All verbs and properties within objects have the appropriate flags, with the user can change to determine its current state. They are also able to assign global names to any object.

Outline of well-known MOOs

LambdaMOO - The MOO.
MOO programming language
MUSH
MUD
CoolMUD
ColdMUD Essays and documentation

MOO at the Open Directory Project
Echo Duet MOO List - Active MOO directory