Short answers, pulled from the story.
Greyhawk is a 68-page rules supplement written by Gary Gygax and Robert J. Kuntz and published for the original edition of Dungeons and Dragons in early March 1975. It has been called the first and most important supplement to the original D&D rules. It introduced new character classes, spells, monsters, and combat rules that became standard in later editions of the game.
The Greyhawk supplement introduced two character classes that went on to become permanent fixtures in Dungeons and Dragons: the thief and the paladin. Neither existed as an official option in the original 1974 rules.
Beholders, displacer beasts, blink dogs, carrion crawlers, and lizard men all made their first appearance in the Greyhawk supplement. These creatures went on to become some of the most recognizable monsters in the Dungeons and Dragons game.
Greg Bell provided the illustrations for the Greyhawk supplement. The cover image of a sword-wielding warrior is a copy of a character called Dax the Damned, drawn by Esteban Maroto for a 1974 issue of the magazine Eerie. Bell sometimes copied comic book figures when Gygax contacted him at the last minute for artwork.
Gary Gygax chose not to publish details of his Greyhawk campaign world in 1975 because he believed new players would prefer to create their own worlds. He also calculated that publishing thousands of hours of campaign material would not adequately repay his investment, and that revealing his secrets would force him to build an entirely new world for his own players.
Greyhawk was published in early March 1975, designated Supplement I with the product number TSR 2003. It was already in production when TSR co-founder Don Kaye died in January 1975.