There are many different types of role-playing games. The PC-based RPG-s of today aren't an original genre; they are derived from board-based or real-life games that have been popular with children and even grown-ups for a long, long time. Another RPG might consist of costumed participants recreating a medieval battle with padded armor and heavy weapons. At their nucleus, role-playing games are a form of interactive and mutual storytelling. The original form of role-playing game was the fantasy war game, inspired by brave knight and witchcraft lore and using minute figures and scale terrain grids to demonstrate action in a way similar to that of strategic war games. Role playing games are as old as time itself. Every child that picks up a tree branch and pretends to be a great warrior is engaging in a role playing game. The system however wasn't formalized until the eighties with the invention of table top, rule based role playing. Then the real explosion for this genre emerged with the advances in computers, and the creation of video games. Today top 10 rpg's are some of the fastest selling titles on the market. However, many of these games are actually online, and free to play whenever you want.
The most basic free role playing games are word based. In the case of computer generated worlds, a scene, a setting, enemies, NPC's, treasures, weapons, and everything else that exists in the world beyond the character is generated by the programmers who create the game. Everything is automated and robotic, except for interactions between the players. In the case of user generated worlds everything, or almost everything, is generated by the users. There may be some scenarios or scenes that are imagined and instituted by more organized groups, but in general the nature of the game is based on interactions between players. This allows for a lot of creative interaction in a media of writing that is generally quite personal.