Like what is it?

Role playing is pretending you're somebody or something that your not.

You cannot be a supermutant in reality but you can role play one, you cannot be an Enclave Scientist in F3 (without mods) but you can prentend you're one.

It's really not hard to understand, you simply fill a roll in a story of some kind, like any of the other players, and depending on how the game is set up, you can go about being a character as you please, doing whatever it is that character might do, or you follow a story to it's ultimate goal which can be a whole range of things: Beating back the opposing forces, setting out on an adventure for something some character has hired your created character, to find or steal, etc.

Usually on here, in the Fallout section, there's two types of RPs. The Open World types, where you create a character following the GM's (Game Master's) Character Sheet template, and go about as whatever type of character you made, free to do anything you like, including creating branching narrative stories that others can fall in and out of, creating branching narratives of their own as well.

Then you have RP's which have a bit more narrative and direction to them by the GM. Usually these sorts of RP's entail you once again creating a character via the given GM's character sheet template, and being apart of the group set out or whatever, to do and accomplish something. Usually these RPs are more linear and you're on a set path from Point A to Point B, and everywhere in-between. It could be an RP where you're taking part as a NCR Trooper, fighting Caesar's Legion in the Mojave, or vice versa, or playing a Brotherhood of Steel Paladin/Knight, fighting the NCR. There's basically an unlimited amount of stories and narratives to create, and it's all up to what the GM creates.

Your duty, like any of the other players in an RP, is to help give it more of an interactive breath, than just being someone making up a story and writing out everyone's character. You're basically tasked with being one of the characters in the RP, reacting and so forth, to what's happening in the story. In Open World RPs though, reaction to other's actions or reactions, isn't always something needed because you might be on one end of the city or wasteland, and they're on the other doing something completely different.

There is however, another type of RP that incorporates an Open World and Linear storyline, which is what I usually make, and has been made every once in awhile. One example (which I didn't create, but it's pretty rememberable), if you bother to fish through the pages for, is an RP called The Warren. This type of RP had it where the players created a character or characters, of any sort they wanted, and it really didn't matter as long as it made sense (which it has to make sense in any of these type of RPs.) Anyway, these characters were free to go about as they wanted, doing whatever they wanted, but there was always a certain group with a certain agenda, and gradually everyone had a part in it that mattered. In the Warren, it happened to be about survivors of the Great War, coming out of their shelters and whatnot, coming together to rebuild and make one of the first settlements in the new world/wasteland, where the players also became the founders and legends of the settlement.

That type of RP allowed you to do pretty much whatever you wanted, but there was always a main narrative and story to what was going on that everyone had to adhere to and be apart of, unlike my first example where people were free to adhere to whatever they wanted, and create stories wherever they pleased within reason, and didn't necessarily have to work with other players or whatever, to do it.

I think Dr. Strangelove pretty much explained it. :]

It's, basically, just making a story with a group of people, instead of yourself as a single writer. The GM's(game-master, or DM - dungeon-master) the "storyteller" that announces what happens and the like throughout the RP, though some are completely open and encouraging of other people's ideas/events/contributions, and the players all play a character or characters that participate within that storyline. With exceptions of limitations and restrictions a GM might make, you can do anything with that character to create your own events - where you're most likely not allowed, without permission at least, is creating events that they didn't cause. Such as a huge group of ghouls attacking the entire town in the hundreds. Because it effects everyone. Your rights end where another person's start, yes?

Which means that, yes, you could have an explainable, big group of ghouls attack you (your character, specifically), but not everyone else.

Going to different sites with different systems have different common types of RPing, and the way RPing works. It's all got advantages and disadvantages.