A role-playing game (RPG) is a game in which you assume the role of another character. That is, you play as if you were someone else. Sometimes it can be kind of like acting.
There are many different styles of games that all bear the title "RPG." For example, Squaresoft's acclaimed Final Fantasy series is considered a collection of RPGs, but my friend maintains that they're strategy/combat. The fact is, these days almost any story with a halfway-decent plot is classified as an RPG.
Metharism, however, is of a very prue variety of RPG. I say pure because in Metharism, you have exactly as much control over the situation as your character would if what happened in the game happened in real life. You can choose to do (or, rather, to attempt to do) absolutely anything.
Obviously, there's no way you could do that in a computer game, because a computer can't improvise. If I made a computer game and told you that you could do anything, and you did something I didn't expect you to, the game would crash. For this reason, Metharism uses what is called a Gamemaster. A Gamemaster is a human who runs the game and controls everything except the players' characters. If you do something unexpected, it's the Gamemaster's job to make up something appropriate. This makes the game remarkably like real life for the players, and creates a style of play that cannot be had any other way.
The things that happen in Metharism, of course, couldn't actually happen in real life. Metharism takes place in a fantasy world, which basically means a roughly medieval timeframe with magic thrown in for good measure. I've also added a dozen sapient races besides humans to keep things interesting.
It is preferable to sit down in the same room with all the other players for a good chunk of time if you want to play Metharism, but in a pinch, you can organize a game using any decent Internet chat program. Players roll dice to determine whether certain actions succeed or fail, and the Gamemaster keeps the game moving along.
If all of this sounds interesting to you, you should explore the rest of the web page. If all of this went right over your head, you can take a look at the General Mesarthim FAQ or ask me for clarif