All content except avatars and attachments is stored in the database.
There is no easy way to export just the nodes, categories and forums.
You can either do a full import and then delete all threads, posts, members, etc. (which isn't really advisable), or you will just have to set them up again.
Ideally if you had set everything up and taken a backup before importing/opening the site then you could have imported that clean backup into your new site.