Have you ever watched someone do something in a movie and thought, "If that was me, I wouldn't be that dumb"? Ever read a mystery novel and thought you could solve it better than the characters did? Ever read a book and wished you could be like the characters? Ever imagined exploring or living in a different world, whether it was something you saw, read about, or just dreamt up? That's what role-playing is about.
Role-playing games are about imagination. When you play a role-playing game, you imagine that you're a character in another world, and you get to decide what that character does.
In any role-playing game, one person is responsible for creating the world and all the people your character will meet. That person, the game master, tells you what you encounter, and what the results of your actions are. The game system has rules that determine what your character is capable of and how likely you are to succeed at anything you attempt. Usually you have a chance of success and you roll dice to find out whether your attempt succeeded. For example, if you want to sneak into a room without being noticed (perhaps you arrived late for class), you would probably have to roll to find out if anyone noticed you. Part of the game master's job is to apply the rules to determine what those chances are, and what the consequences are if you fail. Sometimes the rules of the game also describe the world, so the game master only creates situations, and sometimes the rules can be used in many different kinds of worlds, so the game master creates everything about the setting.
There are other players, too. Usually all the characters—one for each player—are cooperating together to achieve some goal, but sometimes each character has their own goals and getting people to work together is a struggle of its own! Between the game master and all the players doing their own things, no one knows what's going to happen. And that's most of the fun!
Playing a role-playing game is kind of like writing a collaborative novel, or doing a play-reading without the script. There's another kind of game that's more like performing a play without the script. That's known as a Live Action Role-Playing game, or LARP. The basic idea is just the same, except that you do what your character does instead of simply describing it. Of course, some kinds of actions have to be dealt with using rules and descriptions instead. If your character has a sword and you want to attack another character, you don't hit them with a sword. Instead, you might stop and roll dice or compare abilities to find out if you succeed. Or you might play out the combat with "boffer" weapons—harmless, padded props—depending on the type of game. Either way, the rules determine who wins. Live Action games can be very intense. There's a big difference between being told that your character is being chased through a dark forest by a giant—and being chased through a dark forest by a giant!
The other difference between regular role-playing games and LARPs is that in a regular game, the game master plays all the personalities and monsters that the characters will meet. In a LARP, it's hard to play more than one person at once. Instead, LARPs usually have players and cast (also known as staff, non-player characters, or NPCs). Some of the cast write the plots, design the world, and determine what's going to happen. All of them play the people and things that the players' characters will meet.
Both kinds of role-playing games come down to the same thing, though. They are games of imagination, of make-believe. They let you imagine being someone you never were, doing things you've never done, and experiencing anything imaginable.