What is multi-boxing? 

Put simply, multi-boxing is when one person is in control of more than one character (or toon) at the same time.


Why would someone want to do this?

Varying reasons.
The most attractive of which is probably taking advantage of the recruit-a-friend bonus. Getting triple-exp on everything that they do for their first 60 levels is nothing to be sniffed at. Even past level 60 when the triple-exp wears off, two or more toons questing together will be able to get through most quests faster than a lone toon.
Another thing that some multi-boxers do if they have 5, 10 or 25 accounts is try to take on dungeons and raids on their own, which is an... interesting... meta game.
Some people even use multi-boxing for PVP, but I've no experience of that so I can't cover it in this guide.


How is Multi-boxing done technically?

While the name "Multi-boxing" itself originates from the concept of having more than one computer (or box) running at the same time, unless a multi-boxer is trying to control more than five characters the easiest setup is to only use one computer.
Multiple instances of the WoW application can be run at a time on one machine, so the usual multi-boxing method simply involves opening WoW more than once. Obviously a multi-boxer must be running in windowed mode to do this.


What does a multi-boxer need? Is it expensive?

In case it wasn't obvious, multi-boxers need one account per toon. So, depending on how many toons they have and what level they take them to, they may need to throw Blizzard a bit of money at this.
If someone just wanted to get triple-exp up to level 60 then abandon their alt, then assuming that they can hit 60 in their free month plus 10 days then all they're looking at buying is a copy of Vanilla for about £9.
But leveling a five-man-band to level 80 is going to mean 5 copies of Vanilla, tBC and Wrath, and (as this will probably take more than a month and 10 days) five lots of subscription fees.
Then there's heirlooms, which will cost the multi-boxer £15 per fresh account to move over if they wish to use them.
Oh, and if someone wants to stop multi-boxing and put all of their toons onto one account, then we're looking at another £15 per alt.
All in all I think you can see why Blizzard doesn't exactly want to ban this hobby...

How about technical requirements? What spec do you need?

A multi-boxer will also need to look carefully at their setup.
Two copies of WoW can just about be run on a fast single core CPU, but a dual-core processor is probably a sane minimum requirement. Five toons playing at the same time and a quad-core CPU is really going to be needed.
Oh, and a somewhat-powerful graphics card is going to be in order as well.
A second screen is very helpful, but a multi-boxer could work around not having one.
Lastly, I hate to say this because I do love Macs, but it's more difficult to do this sort of power-user stuff on a Macs, so I think that most multi-boxers will be using Windows.


Wait, so does Blizzard allow this?

Yes, they do, but remember that the usual Terms and Conditions apply; i.e. every action that a player's toons take must be caused by a button that they've pressed, and may not be caused by a program that is trying to play the game for them. Blizzard sets these rules to stop people trying to exploit the game, and no-one's arguing with them as they stop the game from being spoiled.
But this does have important implications for multi-boxing, as it means a multi-boxer can't just play one toon while a computer makes the others play automatically.


Um, so if bots aren't used then how does a multi-boxer even play?

Well, first aware that it doesn't have to be complex. Someone could just control one toon, have a second permanently on /follow, and call themselves a mutli-boxer. I know this might sound stupid, but they'd still be able to get the triple-exp from the recruit a friend scheme, so it's not *that* stupid...
In fact, most multi-boxers can really only cope with moving one toon at a time, so most multi-boxers choose one toon as their "main" which they move around, and leave the rest of their toons on /follow on the main.

However the point of multi-boxing is to have more than one toon actually doing something. To achieve this, multi-boxers set up their keyboards so that different key-presses are sent to different windows.
For example, let's say a multi-boxer has two characters, a Mage and a Priest. The "1" button on their keyboard might cause the Mage to cast a fireball, and the "2" button might cause the Priest to cast Prayer of Healing, healing both characters.
That was still an overly simplified example and a real multi-boxer would set up more moves than that; but it should be enough of an example to show how multi-boxing works.


Wait wait wait, how do multi-boxers send key-presses to different windows?

There are multiple programs out there. Some you have to pay for, some which are free. Some provide you a nice GUI and guide you through the setup process, some you need to set up by writing a bit of macro-style code.
I use autohotkey for my multiboxing, partly because it's free and partly because I'm happy writing the macro-style code.


Any limitations on how multi-boxing works?

There's a huge limitation in that a multi-boxer must divide all of their time up between managing their alts. Any action a multi-boxer cannot macro they are going to have to do manually, which means that whilst they are taking full manual control on one toon, their other toons are doing nothing. Positioning toons exactly and using certain spells cause this problem.
Another major limitation is that there's no good way for a multi-boxer to have melee DPS on /follow, as the gap between the followee and the follower is too big for melee range.


How to multi-boxers do whole dungeons by themselves?

There are two main methods. The first method is to have a team full of shaman: I'm told that this does work in some cases ^_^;;
Otherwise it's more or less the same principle as normal; a multi-boxer has a tank (which must be their main, as it is melee), three DPS (which must be ranged DPS, as they are on /follow) and a healer. (A multi-boxer can have two healers for safety, but many bosses are a DPS race and a team does need 3 DPS in those cases.)


What sort of team make-up do multi-boxers have?

Depends on what their goals are.
Multi-boxers who have dungeons/raids/PVP in mind often use the same class more than once in their teams, as toons with identical moves are easier to manage.
My goals are to level up a lot of toons, so I use a mix of different classes.


Can someone use multi-boxing to help fill raids?

In a word: no.
In more than one word: a multi-boxer's alt's healing/DPS really suffers when being multi-boxed. The multi-boxer would really drag the rest of the raid down behind him or herself; it really just wouldn't work.