Volunteer moderator - usually long-standing and trusted community members who keep conversations flowing, welcome new members as elder statesmen, model the behaviour you want in your community and occasionally flag rule breaches.
Moderator - A paid position, generally just there to monitor conversations for risk and rule breaches and take the appropriate action.
Community Manager - Responsible for the communities health and growth, devising and implementing strategy, managing volunteers, member acquisition, initiating conversation during quiet periods and lots more!
IT or Platform Manager - May be a separate role to manage the technical side of the online platform, skins and required settings.