So, it's kinda funny that I've got a few dozen articles into this site before actually addressing what it means to be a Dungeon Master when playing D&D. Sure, a lot of you checking out this site will have already done plenty of DMing and are just here for resources or tips to improve your skills - but for the absolute beginner, you might not even know what the DM is or does. This post is for you!
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Storyteller
So, different people view D&D differently. Some may see it almost as a computer game, where you beat enemies, gain experience points and level up your characters. That's all well and good (and I don't want to disparage anyone who sees D&D like this - you do you!). But for me, I always think of D&D primarily as a storytelling game. More specifically, it's a collaborative storytelling game.
As such, a major role of the DM is that of storyteller (and, also, as one who facilitates the players' opportunities to contribute to the story). In short, you will tell the players the world in which their adventure happens (while they will tell you what their characters do within that world). In a good game, the world you present to them will change according to what their characters are doing. And in a really good game, their depiction of their characters will also change depending on what happens in the world you present.
What does this mean in practice? Basically, you will describe the surroundings, events, and people that your players' characters come into contact with. You will describe any combat encounters they have, roleplay any interactions they have with Non-Player-Characters (NPCs), and explain any relevant plot points that they may discover along the way.
Depending on the style of campaign you want to run, this can involve lots of prep (if you want to have a good grasp on the possible ways your players might take your campaign) or it might involve less prep (if you're happy to improv things and take the adventure in different directions on the fly).
For the absolute beginner (which, if you're reading this, you probably are) I'd suggest doing a reasonable amount of prep before your first session, so you feel comfortable with the world you're presenting. I'd also suggest checking out these beginner tips before you start your campaign.
Rules Arbiter
There are LOADS of rules to DnD. While this might be pretty daunting to some (it certainly was to me when I started DMing!) you need to take to heart that very few people actually know all the rules off the top of their head, and that most rules aren't actually that important. You don't need to know every rule to have a fun session (in fact, some of my most fun D&D experiences have come from disregarding the rules!). Seriously, focus upon just this one rule, and you can't go that far wrong!
All that said, you will need to follow some sorts of rules when you play (whether they've the official ones you'll find in the Player's Handbook(*) or otherwise). And part of your job as DM is to be the arbiter of these rules.
Is what your player wants to do possible within the world you've built? If so, will it require a persuasion or a deception role? Should the DC (difficulty class) of that check be high (for something difficult) or low (for something easy)? Is the strange way a player wants to interpret their spell acceptable/ in the spirit of the game you're running? You get the idea.
There'll be dozens of situations like this, every session - where you'll have to make a judgement call of the rules to the best of your ability, in order to keep the session and the story progressing. Some players will take more or less umbrage if they think you're not sticking to the rules as written - but it is all part of being a DM.
I find it helps to raise the issue during a Session 0 - where I tell my players that I have a fairly loose interpretation of the rules, and that I prioritise storytelling over sticking to particular rules too vehemently. That way they get an idea if the kind of campaign I want to run is for them, and (hopefully) it heads off any rules grievances before they occur. Win-win.
Creator
Linked to the role of storyteller, the DM also acts as creator. Not just of the worlds and NPCs etc (but definitely that!), but also often as a game designer - figuring out how to make the plot points you want to include in your story work practically.
This can be a bit of trial and error regarding game mechanics etc. How would you run a drinking contest, or an archery contest, or a game of hide and seek within your game? Whatever it is, you are the one who decides how to turn your ideas into a workable scenario for your players.
The amount you have to do this will depend on whether you want to run a pre-written or homebrew adventure. When running pre-written stories, most game mechanics for different scenarios should already be figured out and provided for you. With homebrew, though, you are your own master, and can figure out how you want things to work!
Organiser
Probably the least fun of the DMs roles is that of organiser. Sure, this doesn't technically have to be your responsibility, and any member of the party can act as the one to propose the date/time/location of your sessions - but often this does fall to the DM. I guess because they put by far the most work into a campaign, they are better placed to know what frequency of sessions is workable etc.
In my experience, lack of consistency is the biggest killer of campaigns. If you're leaving it weeks or months between sessions then players lose their enthusiasm, everyone forgets the story and in time the motivation to play is lost. Therefore, you should take the role of organiser seriously.
Try to get a date in everyone's diary for the next session when you're together. Let people know whether or not you'll still run sessions if one or more player can't attend, and let people know anything they'll need to do before the next session (levelling up etc).
It can be a bit uncomfortable to have these conversations at times (particularly if there's one individual who is causing scheduling problems for the whole group) but in the long term, taking the role of organiser seriously will help your campaign stay on track - which should enable both you and your players to get the most out of it!
So, there you go - now you know what it means to be a DM, there's no excuse not to try it! Plan a session, get some friends together, and give it a go!