Drupal comes with a forum plugin. In that sense the Drupal Forum exists as the contents of a division (in their case a node) or <div> element on the page.
Most forum packages, like phpBB, myBB, Phorum, Vanilla, SMF etc want to exist as an independent page that generates its own html, head and body tags.
If you want to embed those forums inside your own CMS it isn't easy.
You might think you could simply erase html, head and body generation from what ever templating system that forum uses. But then cookies are lost which keep track of too many system critical things. That's not to say hacking an open source forum so it can be embedded inside a CMS is impossible. It has been done. But it does get ugly. The real culprit (from what I've found so far) is the use of conditional
.... page branching. If the forum codes in question do not do conditional output branching it's generally not so hard to meld the forum's cookie generation stuff into your own CMS.
I need an embeddable forum. I'm willing to invest some hacking time. But I'd like to start out on a well-chosen package where the code strikes a good balance between rich functionality and small code base. I don't want to try writing a forum from scratch and I do want built in support for banning spammers, deleting their posts, etc.