Administrators
Administrators are the people in position with the most power, but also with the most responsibility. They are responsible for the well-being of the site, the enforcement of the rules, and for keeping the site a fresh, fun place that people look forward to joining. They handle disputes and pass down final judgment in those areas.
Administrators have the capability to DM.
Moderators
Moderators are the assistants of the administrators. They handle most of the work that administrators are too busy to deal with or need help on.
Moderators do not have the capability to DM unless specifically given permission by an administrator. Moderators given permission to DM by an administrator are to be brought up to other administrators.
DM/GM
The position of DM is considered advisory. The only power they have is to monitor and manipulate the game of a few players. DMs do, however, have the power to offer their opinions to administrative decisions.
Administrators are DMs, but DMs are not administrators or moderators.