In mainstream media, roleplaying as a whole tends to have a very bad reputation. It's been accused of everything from making teenagers antisocial to turning them into mass-murdering Satan-worshipping neo-Nazis that eat kittens for breakfast and trip old ladies in their spare time. Naturally, none of this is true, at least not about roleplayers as a whole.
Roleplaying is difficult to define exactly, except for the obvious "Roleplaying is when you're playing a role." More specifically, it's where you take up a character and play out the situation how you think that character would act. Think "improvised acting" and you'll have a pretty good idea of what roleplaying actually is. It's also been compared to playing "pretend" with your best friend when you were little, except for, well, grown-ups.
With such a broad definition as simply "playing a role," there are a myriad of forms in which to RP. However, most of them fall under the following three:
- Computer-based RP, such as a chatroom or an MMORPG,
- Pen-and-paper RP, such as Dungeons & Dragons,
- LARP, or Live Action Role Play, which best fits the "improvised acting" definition.
Not all roleplayers choose to take part in all three, although some definitely do.
As mentioned before, RP has gotten a really bad reputation in the media, particularly Dungeons & Dragons, which was subject to an intense hate campaign by many Moral Guardians. Several books were written on the subject, as well as a movie made, most of which aren't very well-known. LARP and chatroom RP haven't seen as much grief from the media due to the obscurity of LARPing and the fact that there are many, many better things to complain about on the internet than RP.
Of course, to RP, one needs to act as a character. A significant amount of RPers roleplay a character from the favorite book/TV show/movie/whathaveyou (Journal Roleplay in particular revolves around this,) but many still come up with completely original characters to use. In an MMO, expect the player to use their in-game avatar.