Multiboxing, the process of one person playing multiple characters on multiple accounts at one time, usually by the use of multiple computers (thus the term) and macros that can be activated on all accounts by the push of a single button, has most recently seen coverage here on WoW with our 2-man Karazhan report. The act of multiboxing is one that has been the subject of some debates, mostly centered around whether or not it violates the EULA. Those in favor of multiboxing can breathe easier today, as Blizzard poster Belfaire has stated in no uncertain terms that Blizzard has no problem with the practice in a post on the customer service forums.
In short, he says that the advantages of multiboxing are no different than the advantages offered by normal grouping. Since multiboxers can be damaged, feared and CC'd as easily as separate people playing separate accounts, and since they can't do anything the same amount of characters couldn't do when played by different people, there is no reason to consider it an unfair advantage in PvP or PvE. He also answers quite a few specific questions posed by thread starter and multiboxer Velath that clarify why Blizzard accepts Multiboxing and does not consider it an exploit or an unfair advantage.