It may help if you read my node on Freeform Online Role-playing Games
Back? Here's a quick refresher-- in the game, you can write whatever you want, but you have to follow natural narrative rules. Given what is written, you have to continue writing is such a way that makes sense.
The problem arises when you introduce magic into the mix. Fantasy has been a staple of roleplaying games of any sort, diceless or using lots of dice, since just about the beginning. Dungeons and Dragons, after all, made the genre famous.
The reason this is a problem is that if you were to pick any two random fantasy novels off the shelf of your local library, you would find entirely different descriptions of magic. The method in order to make it work, how powerful it is, how you stop it... and in novels, this works because a single work of fantasy needs only be internally consistent. No one expects Robert Jordan and Piers Anthony to collaborate on how magic works, and quite frankly, I don't think anyone wants them to either.
But with chatroom roleplaying, where everyone can be anyone they want and yet still must interact with each other, problems arise. Say one character casts a spell at another character. In the first character's magic system (say, from random fantasy novel A) spells are fearsome eldritch works of horror, and nothing but silver can block it. However, the second character is using a magic system in which magic is considerably more common, and a "magical shield" can be created to block any spell.
Which system takes precedence?
Since for a while, I was given the reins of such a game, in that I was a moderator and arbiter of the rules, I had to give this matter very serious thought. How could I be fair to everyone? The solution was to declare that all magic must be explained somehow in terms of things we humans can understand... because in explaining exactly how something works, you can see how it works with other things.
Now, this may take a bit of mystery out of magic, but it's a necessary thing when rituals and incantations meet up with a Staff of Fireball +5.
For those that didn't want to take the time, I wrote up a set of rules for generic magic. Almost ANY fantasy magic system can be made subject to these rules. Of course, once I wrote the rules out, people started using their creativity to construct systems that deliberately did not follow these rules and yet were explainable to mere mortals. Indeed, I invented one or two such systems myself. But this one works for the majority of what you'll come across.
Magic is commonly considered to be a kind of force. Unlike most forces, it is very chaotic in nature.
But, unlike most known forces, it is much more versatile. It is chaotic in nature, but not simply utter chaos; After all, if it were truly utter chaos, no wielder would be able to get it to do what they want it to, it would do something entirely random (which of course, in some cases, would mean doing just what they wanted it to, but at random).
This force called magic is something which requires a proper paradigm in place to use. Methods and results vary dramatically, but they all follow similar sets of rules, because each method basically performs the same function: a bridge between Newtonian physics, and sheer chaos.
1. Magic and Force
2. Magic and Effects
Magic and Force
Magic, once converted from chaos into something you can use, requires two things: a point of entry, and a line of attack.
The point of entry is the location from which chaos is initially converted to order. Almost always, this location will be the location of the caster, or the magical artifact. Only in very rare circumstances will the point of entry be located elsewhere, and even if this is the case, the distant point of entry must be triggered in some manner, possibly by a more conventional spell with the point of entry at the caster.
Once the energy has entered into an ordered system, as to accomplish a task, it needs to be aimed somewhere. This is what I am calling the line of attack. When you attempt to enchant a target, magic is called into being at your location, shaped, and then sent in the direction of the target. This line of attack, however, can be manipulated by outside forces, in every manner that any force can be affected under standard laws of physics. The only difference is that only magical forces can effect magical forces.
For some basic rules of force, here's a quick refresher course:
Rule 1: It takes effort to make magic, and to move magic.
What this means to you: In the simplest terms, magic doesn't just happen. Something needs to cause it.
Rule 2: A line of attack will tend to go in a straight line, unless effected by something else.
What this means to you: If there's a magic-blocking wall between you and your target, you're not going to effect the target.
Rule 3: It takes less effort (see rule 1) to change magic, than to stop it. While the line of attack will be a straight line unless effected by something else, it can be effected by something else. Pushing it to make it curve will work.
What this means to you: Magical defenses tend to require a lot of power and effort (see rule 1) because the objective is to stop the magic outright. Magic that effects magic, however, could be used to push the line of attack to the side, deflecting it, instead of stopping it.
This last one has a lot of interesting ramifications to roleplaying, as it means that metamagic is often more effective than anything else. Instead of dispelling, try shifting the target to someone else instead. Or alter the spell slightly... put all that energy to YOUR use, instead of the other person's.
But anyway, it's time to move on to how the energy actually does stuff.
Magic and Effects
Ok, so magic is a force. How does that force interact with things in the real world? Well, the specifics depend on the magician (or wizard, or sorcerer, or whatever you wish)... but in general, there are things to consider.
People tend to focus on the easiest way to accomplish any task. Since magic takes effort, most people will wish to use the easiest possible method to accomplish a goal. This usually means making a small change in the world, and letting natural physics do the rest.
Spells tend to do a combination of one or more of the following:
Apply physical force.
Apply spiritual force.
Move something long distances (see Tesseract)
That's it. It's just how you apply these four effects that make up spells. Patterns are used frequently.
For example, your standard fairy tale witch is often found turning handsome princes into animals of whatever sort, be it frogs or swans or anything you can think of. What is the spell doing? Well, the first thing it has to do is apply a spiritual force to the soul of the target, to make sure that the target doesn't die from having his form changed about. The second thing it does, is apply a physical force, by rearranging the molecules in the target to a new pattern.
Not that the above description really matters much in game, but it's good to think about how these things work. For example, maybe that's too complicated for you... perhaps it would be easier to just apply spiritual force and stick the guy's soul into a bunny rabbit, then "tesser" the human body elsewhere, and the rabbit body here? If a sufficiently clever foe is trying to play a game of mental acrobatics with you, you'd better know how your magic works in order to know what WOULD counter it...
Fireballs are simpler things. That's just a matter of changing temperature, ie, increasing it to the point that the air burns. Or, for a less powerful fire bolt, first the magic gathers together some sort of flammable material, sets that on fire by changing the temperature, and then flinging that.
Which means that in most cases, fireballs can be combated by purely mundane means, and that antimagical areas won't actually stop one of these... unless you want to sustain the fire with magic, in which case antimagic would help.
Now, all this is a lot of blathering for what is often a very simple part of the game... I say I can cast spells, and so I make magic things happen. It's only when you get into direct conflicts with other players that these rules really matter. When both players are in harmony and accord anyway, then all of this is just an intellectual game.
Which really, is quite a lot of fun.