Game Masters, or GMs, are server's host employees. GMs are the overseers of the server and watch over a server to solve problems, disputes, and sometimes punish people disobeying the rules set forth. You can create an in-game petition (usually called a "ticket") to alert the GMs to certain problems you are experiencing. Typically, GMs only initiate action when a ticket has been filed by a player, though scattered exceptions to this have been noted.
Sometimes, the term GM or Game Master has been used to refer to any gm/admin who works on the server. To avoid confusion, developers, and others with specific jobs are referred to by those titles, and official blizzard posters are referred to as "blues" or "blue posters" on the official forum. Game Masters are NOT to be used for help with quests, asking them for money, asking them for gear, etc
what apperance a gm should have:
GMs interacting in game will have "Blizz vs.gif" prefixed to their name in the chat interface to prevent GM impersonation. GMs showing themselves to players will always start on the so called "GM Island" as soon as they create the character. It seems that this island is not of any other use as those GMs seen on there have not moved at all until they teleported away. GMs are allowed to choose the name they want to use themselves. They don't usually have characters on every realm, if they have to meet players in the game world it might take some time until they have created and prepared their character. GMs showing themselves inside of the Game World are still using another IM-like client to whisper with players. If you ever happen to meet one in game, you will notice that his character name has got some slight differences to his name in private conversations. GMs still appear in the game to some players. They have got the possibility to change their displayID, for example into that of a pit lord.[2] One interesting fact about this is, that they do have spells for that instead of using the console. It seems that not every GM, if any of them at all, has full console access. There is a special tab in the spellbook for blizzard spells, that contains every spell a GM could ever possibly need. (e.g. spawn/despawn/teleport players/appear to players and so on). GMs appear to be prohibited from using their position to affect any in-game characters they have a connection with, including their own, or those of friends and family. GMs are not allowed to moderate a realm in which they have their private account characters, or if there is a conflict of interest. GMs are not server-bound.