Risus is not confined to a selection of pre-defined character-classes, settings or abilities. Characters are described via clichés such as Witch Doctor, Gambler, Musician with a number of dice assigned to each cliché that are then used in challenges that relate to the appropriate cliché.
Risus has a very minimalist rule system that breaks down all actions to their core function. Risus comes with a super short, four page rulebook (available in many forms and formats) that's easy to read, includes examples and has a bit of humor thrown in to make the process or learning more enjoyable.
A large amount of adventures, artwork, characters, clichés as well as optional rules can be found for Risus. An excellent resource for all aforementioned things as well as many others is Risusiverse, a fan-made website of all things Risus.