Short answers, pulled from the story.
Tactical Studies Rules published the Blackmoor supplement and introduced the Dungeon Master in 1975. This publication changed the landscape of entertainment by shifting tabletop gaming from strict wargames to collaborative storytelling.
The Dungeon Master relies on the Player's Handbook, the Dungeon Master's Guide, and the Monster Manual to conduct duties in the Advanced Dungeons & Dragons system. These three hardbound books contain the complex rules that govern character interaction, dice success, and monster behavior.
The Dungeon Master improvises when the rules fail to cover a situation by deciding instantly whether to allow creative solutions or roll dice to determine the outcome. This skill defines the difference between a rigid referee and a dynamic storyteller.
The Dungeon Master prepares a customized version of the adventure that may require several hours of work to design decision trees and create responsive scenarios. This preparation involves anticipating player actions and building a world that maintains realistic continuity of events.
In the Faiths and Pantheons sourcebook, the Faerunian Overgod Ao answers to a superior entity insinuated to be the Dungeon Master. This meta-fictional connection elevates the role from a game organizer to a cosmic force that exists beyond the game world.
Some gaming companies use more generic terms like Game Master or Storyteller to avoid trademark infringement since the title Dungeon Master was invented by Tactical Studies Rules. Other titles such as the Keeper of Arcane Lore or the Hollyhock God reflect the specific genre and style of their games.