I found a good description of the difference between a forum and a wiki.
http://support.etouch.net/cm/wiki/?id=65089
Forum:
“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.”
Wikis:
“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.”