I am more focused on the backend server design than actually playing a game. :)
World of Warcraft was first announced by Blizzard at the ECTS trade show in September 2001. Development of the game took roughly 4–5 years, and included extensive testing. The 3-D graphics in WoW use elements of the proprietary graphics engine originally used in Warcraft III. The game was designed to be an open environment where players are allowed to do what they please. Quests are optional and were designed to help guide players, allow character development, and to spread characters across different zones to try to avoid what developers called 'player collision'. The game interface allows players to customize appearance and controls, and to install add-ons and other modifications.
From wowwiki about Realms:
A realm is an instance of the World of Warcraft (WoW) game world. Realms are hosted on physical devices called servers (for this reason, the terms "realm" and "server" are often used interchangeably).
To play WoW, you must first choose a server (realm) to play on. You then connect to this realm over the internet. Blizzard has servers in the US, Europe, China, and Korea (see Localization).
See the realms list for a list of realms kept up-to-date by WoWWikians.
Why have realms?
The WoW game world is duplicated (mirrored) across multiple realms to prevent any one realm from becoming too crowded (and thus overwhelming the server), to provide for better connection times (by having players connect to local servers), to allow for localized communities based on language (for example, French speakers on realms in France), and to appeal to different play-styles for different users.
Another site with a list of mmorpgs:
Depending on the number of players and the system architecture, a MMORPG might actually be run on multiple separate servers, each representing an independent world, where players from one server cannot interact with those from another; World of Warcraft is a prominent example, with each separate server housing several thousand players. In many MMORPGs the number of players in one world is often limited to around a few thousand, but a notable example of the opposite is EVE Online which accommodated around 20,000 players in the same world as of August 2007 and 51,675 users online in February 2009.
Some games allow characters to appear on any world, but not simultaneously (such as Seal Online: Evolution), others limit each character to the world in which it was created.
Here is a for pay article on mmorpg networking.
We find that MMORPG and FPS (First-Person Shooting) games are similar in that they both generate small packets and require low bandwidths.
In practice, the bandwidth requirement of MMORPGs is the lower of the two due to less real-time game playing. More distinctive features are the strong periodicity, temporal locality, irregularity, and self-similarity observed in MMORPG traffic. The periodicity is due to a common practice in game implementation, where game state updates are accumulated within a fixed time window before transmission.
The temporal locality in game traffic is largely due to the game’s nature, whereby one action leads to another. The irregularity, which is unique to MMORPG traffic, is due to the diversity of the game’s design so that the behavior of users can vary drastically, depending on the quest at hand. The self-similarity of the aggregate traffic is due to the heavy-tailed active/idle activities of individual players.