There is a lot of information here on the forums that can answer most of your questions. The sticky threads listed at the top of each section are a good place to start.

Here is a quick answer. To control multiple instances of a game on a single computer requires software. To control multiple instances of a game on different computers can be done with either hardware or software.

Keyclone is very popular for either scenario. How it works is it broadcasts key strokes to either multiple instances of a game on a single computer or to other computers. I use a single computer right now, but in the passed, I multiboxed with up to 4 computers using hardware only. Not having to move my hands from one keyboard/mouse to another keyboard/mouse has been great. It does require a lot of thought about how to lay out your keys/spells/macros though.

For instance, I use a paladin/4 shaman setup. My paladin obviously has different skills from the shaman and I also want do do things at different times. I have laid out my spells and macros so that when I press 3, 5, ctrl+3, ctrl+5 and so on that my paladin uses certain skills. I have all shaman cast lighting bolt when I press 2. This means that I do not have anything in the paladins hotbar in the 2 slot. I also have the shaman earth shock in a round robin rotation when I press 4. This prevents me from using up all the shaman's shock cooldowns simultaneously.

As to movement, I have all the shamans follow the paladin most of the time with a follow macro. I have unmapped all of the movement keys from the shaman except the cursor keys and my "formation" key. This works well most of the time, but I need to rethink how I'm moving them for situations where the paladin is dead or whatever.

This is just a starter, and the way I do things and is in not in any way the "best" way. My way is probably fairly crude in comparison to some of the other boxers. There are probably as many different methods as there are people, so you just have to find what works for you.

So in general, poke around the forums and read the beginners stickies for more information.