Complex sentence: Shortly thereafter, another listener volunteered to launch the "Big Mouth Fans" web site, a vBulletin message board to give listeners a place to further discuss issues and subjects related to the show.
Split into simpler sentences: Shortly thereafter, another listener/volunteer launched the "Big Mouth Fans" web site. It is a vBulletin message board to give listeners a place to further discuss issues and subjects related to the show.