PB Wikis

I've just been playing around with ML's PBWiki. I started a new page, 'Favourite Music Artists' only to realise that there was already a 'Favourite Music' page which I hadn't seen (was under 'view all pages' on the left hand side of the FrontPage.) I cannot yet discern the difference between a normal internet forum and a PBWiki, excpet for the fact that it enables you to have a forum without having to set up your own internet website.