Simply put, "roleplaying" is the act of assuming a role within a fictional environment. Like an actor in a play or the author of a novel, you put yourself in the shoes and in the mind of a character that is separate from yourself. This can be as simple as assuming the part of an elf in a hack-and-slash MUD and as complex as immersing yourself into the role of an alien smuggler in an intensive, RP-enforced sci-fi MUSH.