From what I understand, outsider = raw = brut art. By definition, it's done by artists who live in isolation from the rest of the art world and have no training; often they are institutional patients who do art for therapy or because of compulsive behaviour that they are venting.
A few months ago, I did some web searches on it, and found it very interesting. Give it a shot if you have a few hours to surf and you'll find some very good (and some very bad) work, lol.
/Karen