A massively multiplayer online game, or MMO or MMOG, is a type of computer game that enables hundreds or thousands of players to simultaneously interact in a game world they are connected to via the Internet. Typically this kind of game is played in an online, multiplayer-only persistent world.
Dragonica is a MMOG and a MMORPG.
MMOGs compared to other computer games
There are a number of factors shared by most MMOGs that make them different from other types of computer games. MMOGs create a persistent universe where the game continues playing regardless of whether or not anyone else is. Since these games strongly or exclusively emphasize multiplayer gameplay, few of them have any significant single-player aspects or client-side artificial intelligence. As a result, players cannot "beat" MMOGs in the typical sense of single-player games. One exception is "Star Sonata," a free-download MMOG located at http://www.starsonata.com/. In this game, you take control of a space-faring vessel, and through trade and negotiation you can be crowned "Emperor" and "win" the game. Nonetheless you'll find lots of NPCs and Mobs in most MMOGs who give out quests or serve as opponents. (see Timesinks)
Most MMOGs also share characteristics that make them different from other multiplayer online games. MMOGs host a large number of players in a single game world where all those players can interact. Popular MMOGs might have thousands of players online at any given time usually exclusively on a company owned server. Non-MMOs, such as Battlefield 1942 or Half-Life usually have less then 50 players online and are usually played on private servers. Also, MMOs do not have any significant mods since the game must work on company servers. MMOs are special in having spawned virtual economies of game items, something that other game types have not yet created.
There is some debate if a high head-count is the requirement to be a MMO. Some say that it is the size of the game world and its capability to support a large number of players that should matter.  For example, despite technology and content constraints, most MMOGs can fit up to a few thousand players on a single game server at a time. Given technology development online multiplayer may eventually become MMOs as well as the average server size increases. However, by then the benchmark may have increased as well and MMO's will host many times even that. Alternatively, if the defining characteristic of MMOs is that all the players must be in a single-world where they can interact, online games with a highly fragmented un-connected sever base are not MMOs.
To support all those players, MMOGs need large-scale game worlds. In MMOGs, large areas of the game are interconnected within the game such that a player can traverse vast distances without having to switch servers manually. For example, Tribes comes with a number of large maps a server plays in rotation (one at a time), but in the MMOG PlanetSide all map-like areas of the game are accessible via flying, driving, or teleporting.
There are few more common differences between MMOGs and other online games. Most MMOGs charge the player a monthly fee to have access to the exclusive servers. The game state in a MMOG rarely resets; what the player earned yesterday is with them still today. MMOGs often feature in-game support for clans and guilds, such as the ability to manage an association with in-game tools.
The boundaries between multiplayer online games and MMOGs are not always clear or obvious. Neverwinter Nights (2002) and Diablo II are usually called online RPGs but are also sometimes called MMORPGs (a type of MMOG).
The popularity of MMOGs is mostly restricted to the computer game market. Online games have not yet gained huge audiences on console game systems. Nevertheless, there have been several console MMOGs, including Phantasy Star Online (Dreamcast), EverQuest Online Adventures (PlayStation 2), and the multi-platform Final Fantasy XI (Microsoft Windows, Playstation 2, and XBOX 360).
Types of MMOGs
There are several types of massively multiplayer online games.
Although not prefixed with MMO, ARGs (Alternate Reality Games) are massively multiplayer, allowing thousands of players worldwide to co-operate in puzzle trails and mystery solving.
Massive multiplayer online computer role-playing game, known as MMORPGs, are perhaps the most famous type of MMOG.
Several MMO first-person shooters have been made. These games provide large-scale, team-based combat. The addition of persistence in the game world means that these games add elements typically found in RPGs, such as experience points. The first MMOFPS was probably 10SIX, released in 2000. Other popular MMOFPS games include World War II Online and PlanetSide.
A number of developers have attempted to bring real-time strategy games into the MMOG fray. Some notable MMORTS games include Mankind, Shattered Galaxy and Ballerium, which is still in development.
Most other MMOs are apparently simulation games, such as Motor City Online, The Sims Online (though this is often called an MMORPG), Ace of Angels, and Jump Gate. There are also games like Second Life and There that derive from the tradition of MUSHes, emphasizing socializing and world-building.
An interesting variation from the classic MMOs is Game Neverending - a "game of social, political and economic actions" which was announced in 2002 and is still in closed beta.
In April 2004, the United States Army announced that it is developing a massively multiplayer training simulation called AWE (asymmetric warfare environment) that is expected to begin operation among soldiers by June. The purpose of AWE is to train soldiers for urban warfare and there are no plans for a public commercial release. Forterra Systems Inc. is developing it for the Army based on the There engine. 
The first type of MMOG was the MMORPG. MMORPGs themselves trace their roots to MUDs, Bulletin board system (BBS) games, and browser-based games. As computer game developers applied MMO ideas to other computer and video game genres, new acronyms started to develop, such as MMORTS. MMOG emerged as a generic term to cover this growing class of games. These games became so popular that a magazine, called Massive Online Gaming, released an issue in October 2002 hoping to cover MMOG topics exclusively, but it never released its second issue.
By applying MMO to a number of different genres, and sometimes to multiple genres at once, game developers, gamers, and marketing departments have created long and obscure acronyms. One humorous example is "SFMMORPGRTSFPS" once used to describe PlanetSide ("sci-fi, MMORPG, RTS, FPS").  There have been a number of attempts to popularize new names, but "MMO" and "massively multiplayer" became gaming buzzwords in the late 1990s and early 2000s. Massive Online Gaming magazine attempted to shorten the name to just "massive online gaming" (or "MOG"), but few used this version. MMO had entrenched itself as a selling point for many games.
Another small controversy exists over whether it is "massively multiplayer" or "massive multiplayer" (without the '-ly'). Some say that massive multiplayer emphasizes the nature of the game over how many players actually play it. Others also prefer massive since they believe that multiplayer is an absolute adjective and cannot be sensibly modified by the adverb massively (e.g., just like "very dead"). Many gamers feel that the issue is trivial.
- Multiplayer Online Games Directory has a listing of massively multiplayer games
- ODP: Massive Multiplayer Online category
- Collaborative weblog about an emerging social phenomenon called Massively Multiplayer Online Game.