g1smd - 9:12 am on Mar 1, 2011 (gmt 0)
The forum software lives in the /forum folder of your server, but do you want users to see "/forum" in the URL they use to access the forum?
If you use a rewrite, users can access the URL / and see the content from the /forum folder.
If you use a redirect, user asking for / will be redirected to the /forum folder and the URL shown the URL bar of their browser will change to show the new location. It is bad form to redirect the root of a site to an internal folder.