Group roles identify what managerial functions group members can perform within the group. In addition to regular members, there are group administrators and group moderators. Group administrators can change group settings (e.g., turn the group blog on or off, turn group forums on or off, change group status from public to private); post announcements; change group avatar; manage group members (e.g., ban members, promote or demote members); and delete the group. Group moderators can edit the Group Details, including the name of the group and group description; edit, close, and delete forum topics and posts; edit or delete group documents; and post announcements.
These are covered in more detail here.