As a Community Consultant for Mzinga, I spend time with my clients talking about the different roles they will need to take in order to have a successful community.  I also spend time talking about the roles of the community members. I talk about gardners, the ones who want to keep things in order; the evangelists, contributing the most content, trying to keep others engaged; the lurkers, they are looking rather than contributing -- we nurture these folks.  They are either listening to or turning into evangelists.  we also talk about trolls, these are the folks who are just there to ruin the party. 

Fortunately, I haven't experienced trolls in any of our client sites.  We have lots of processes in place to keep the trolls at bay, and to remove them before they poison our sites.  Now, people are starting to look into the psychology behind trolls.  Kind of cool to find out why they do what they do.  Maybe it will help Community Managers keep them at bay.

If you want to know more about community roles, here's a great site that covers lots of different roles and other terminology.  It's from a wiki perspective, but most of the terminology translates easily to the general community setting.