A Guild Master(sometimes abbreviated as GM) is the leader of a Guild. The Guild Master unrivaled power/capabilities within the guild to recruit, promote, alter and dismiss members. The GM may also change the guild name, set the charter, forge or break alliances, set member titles and has other capabilities as well.
The first player who creates a Guild is automatically the Guild Master and may only be removed from that position by promoting another member to be Guild Master, resigning from the guild or being voted from office by a vast majority of member votes. If a Guild Master is removed from office by vote or promoting another to the office, they will still be a member of the guild but their membership level reduced to a regular member standing.
Some guilds adopt their own leadership framework for role-playing or other reasons by having a council of leaders, senior advisors, etc., but the Ultima Online game mechanics only allow for a single(both titularly and capabilities-wise) Guild Master slot.