Difference between a GM and a DM?

Question

Terminology question for you; what's the difference between calling yourself a DM (Dungeon Master) or a GM (Game Master)? I've seen both terms on this site and I can't figure out if there's an actual difference between them, or it's just a quirk of either time or preference.

To be more specific;

1. Is this a rpgSE-specific terminology creation, or do the two terms (Dungeon Master vs Game Master) refer to different things across the entire bredth of rpgs?
2. If they are different, how so?

My best guess is Dungeon Master is specific to running a Dungeons and Dragons game, where Game Master is more universal but even there I'm not sure if that's just on this site or everywhere.

Answer 1

Game Master (GM) is a generic term for the person who's running the game.

Some games have their own name for that role — Dungeon Master is the term Dungeons & Dragons uses. It's even a Wizards of the Coast trademark. (Yes, really.)

Some games use “GM” for their actual official name for the role, such as Fate or Dungeon World. Others use a more specific term. We can often use “GM” as a generic replacement for this type of role regardless. Games vary considerably in the roles and responsibilities they place upon the GM, so one game's GM is not necessarily another's, and some games have chosen their term for this role (such as “Master of Ceremonies” in Apocalypse World) specifically to evoke a reminder of those responsibilities.

We didn't invent these terms. Dungeon Master arose sometime in the 70's, and Game Master was already in use before our hobby started: see inky's chronicle of the history of these terms here.

Answer 2

Dungeon Master was the first term in an RPG, used in the original edition of D&D. However, various subsequent games that didn't involve dungeons used other terms. Game Master is used in quite a few, of which Chivalry & Sorcery was probably the first; Call of Cthulhu uses "Keeper," while World of Darkness games use "Storyteller."

Game Master has history, stretching back before the invention of RPGs, in postal gaming.

All of these terms describe the same basic function, and are used generally throughout the field; I haven't run into any that are specific to rpgSE.

Thanks to @Bloodcinder for pointing out that WotC has a trademark on "Dungeon Master," which presumably prevents other games from using it as their term in their own rules.