I found a good description of the difference between a forum and a wiki.
“Discussion Forums (also referred as “Message Boards”, “Bulletin Boards” etc) can be simply thought of as an area where people would just like to initiate a topic or idea so people can start commenting and collaborating around it. The initial idea/topic ( also called as a thread) may be just a single line or a complete descriptive paragraph. But they essentially differ from Wiki pages in the sense that the comments are an integral part of the discussion thread.”
“In the simplest form, wikis can be imagined as a repository of easily editable website pages. Each wiki can have a specific purpose and a bunch of wiki pages that are maintained by the users of that Wiki. Wiki pages typically have comments that can be used for collaborating around that page/topic.”