There are two types of forums in TopCoder.
General forums that deal with community specific topics and are open to everyone: http://forums.topcoder.com/
Note that general forums for topics dealing with graphical design contest are located here: http://studio.topcoder.com/forums
Contest specific forums, which are created for a specific contest and are usually ONLY open to the registered participants. Such private forums protect the intellectual property of the clients as only participants that have signed the CAA are allowed to post/read in such a forum. Participants can ask contest specific questions to the client, and provide feedback about progress of the contest. In addition the client could provide contest specific announcements.