Most people tend to associate RPGs with sword-swinging maidens in chainmail bikinis and doddering old white-bearded mages in robes spattered with owl poop. While the high fantasy setting is certainly the stock background for most RPGs, an RPG is defined not by its content but by its manner of presentation. To be a true RPG, a game must contain three elements. First, it should offer up an interactive story in which the player takes a vital part. Second, RPGs must allow for character growth that's driven by a player's choices or actions. Finally, RPGs must be built upon a system of rules and statistics that are used to resolve the events that take place in the world.
UPD: вообще рекомендую тем кто интересуется историей возникновения PRG и развитием жанра: http://pc.ign.com/articles/633/633762p1.html