Content Management Systems can provide for ad hoc
or structured input of user content, but are not the same as what is commonly called a 'forum' (such as this one) although there seems to be some blurriness where the two are distinct. phpBB
was one of the early products, and there are others like vBulletin
(upon which I think this forum is based). Forum Software Reviews
has a good list of some packages. A Google search for 'internet forum software' produces other good references.