If there's one thing all Massively Multiplayer Online Role Playing Games have in common, it's that they have a hard limit on how many players they can handle, per server.
A game like World of Warcraft which boasts a membership of 5,000,000 subscribers worldwide certainly needs more than just ONE server for all those players, and other MMORPG's are the same, even if they don't have even 10% of the membership.
Every MMORPG designer makes their own design decision as to how players will be split up amongst many servers in order to play the game.聽 Runescape, for example,聽has chosen to allow players to log into any of their "worlds".聽 World of Warcraft, however,聽has chosen a different route, where players must select a specific Realm for their character to exist in, and the player cannot log into any other Realm with that character.
In essence, on a game like RuneScape, you have no real "home" server, although you may personally choose to log into the same server by default.聽 In World of Warcraft, your character actually has a Home Realm, and you cannot stray from that Realm, even though the maps and quests and gaming world you encounter is identical to what others see in their Home Realms.
There are a聽few things that are impacted by this design decision regarding player load balancing.聽 Namely:
Player names and identities
Communication with Real Life friends
Population聽of your Realm
Player Names and Identies across Realms
One of the first things I noticed as an experienced RuneScape player was that hardly anyone - if anyone at all - has NUMBERS after their names on World of Warcraft.聽 Most people have standard gaming names, whereas RuneScape is overrun with "IRC names" that are very difficult to remember at times.
Sure, there are people on World of Warcraft who have to misspell a name to get what they want - Onslaught was taken or the guy didn't know how to spell it so he became Onsolought... but I find that without numbers, people's names are much easier to remember and even聽to interact with.聽 Perhaps it's due to my MUD'ing history.
Of course, if you play in multiple realms, don't expect that Maxwell on Zul'jin is the same guy as Maxwell on Silvermoon... they could be completely different people because they're on completely different realms.聽 This is another reason to try to invent your own player name instead of taking one from history or mythology - it's less likely someone will have taken your name already, when you try out different Realms.
Communication with Real Life friends
Selecting a Realm is a breeze if you're the only one you know in Real Life who plays World of Warcraft.聽 All you have to choose is whether you want to Role Play, engage more actively in Player-vs-Player gaming or whether you just want to go the "Normal", player-vs-environment route.聽 Then you select a Realm name that appeals to you and go from there.
But, if you have Real Life friends and family who play, it's likely you want to be on common Realms and even in the same faction - which takes a bit more planning.
In short, to communicate in-game with your RL friends and family, you need to be living in the same Realm and you need to agree upon whether you're be in the Alliance or in the Horde.聽
If you want to start partying and killing in the same areas right away, you'll have to choose the same race, but if you're okay with playing alone and having communication with your friends and family, you can select a different race from within the same warring faction.
Population of your Realm
With characters essentially "locked" to specific Realms, population handling can be a problem at times.聽 Realms that have been around for a long time and have a lot of players in them can have a lot more lag than Realms which are newer or have less players - especially when you get into highly-populated cities.
At times, Blizzard will look to relieve the load on overloaded servers, to the detriment of underloaded servers, by authorizing transfers from one specific Realm to another specific Realm in an attempt to even out the loads.聽 The once-underloaded server receives a large influx of players at high levels, which may be frustrating for those who enjoyed the sparsity of other players.
Conclusion
The world that the Realms see is exactly the same, but players are "locked" to the Realms they choose, and communication is restricted to those on that Realm and within your faction.
Selecting a Realm is, therefore,聽a breeze if you're the only one you know in Real Life who plays World of Warcraft, but joining Real Life friends and family takes a bit more planning.
Once you get in there, however, it's all fun and games!