World of Warcraft owners will be able to request a transfer to a different realm for their character, according to an official Blizzard statement in the WoW forums. The system will go live this summer, and will require you to pay a fee for each character transferred.
Realms are the names for the particular server that you connect to, with each area of the globe having a certain number of realms for you to choose from. Transferring between these realms is possible now, but only as part of a mass-migration by Blizzard, which can take any length of time.
The new system will allow players to move at the time of their choosing, and to almost any other realm they want. PvP (Player vs Player - servers for massive multiplayer brawling) characters can move to any realm they like, but PvE (Player vs Environment - the natural, quest-based gaming type) gamers are limited to other PvE type realms.
While the charges for the service have yet to be announced, this is just another example of Blizzard's ongoing commitment to its huge MMO. Just imagine what would happen if everyone stopped playing all at once...