Greyhawk was the first supplement published for the original "white box" Dungeons & Dragons. It was published in 1975. Gary Gygax and Rob Kuntz were the authors.

Despite the name, the supplement in of itself was not a tour of Gygax's fabled campaign setting. It was strictly a set of additions: new character classes, new monster, and new rules. Much of the material, no doubt, came out of Gygax's Greyhawk campaign. Hence the name. Kuntz's co-authorship arises from the fact he had long been a co-DM of Gygax's Greyhawk campaign. (Kuntz actually ran his own "Kalibruhn" campaign for many years as well. His fictitious land existed within the greater Greyhawk milieu.)

Greyhawk's cover featured a fighting man facing down a beholder. The inside cover featured a rather familiar image: a defiant lizardman holding a polearm. This image was later shrunk and used as TSR's corporate logo for a number of years (two subsequent logos were the wand-holding mage logo and then flowing hair wizard head that bore more than a passing resemblance to Proctor & Gamble's moon and stars logo).

Greyhawk added the Paladin and Thief character classes. Along with all things Conan-ish, Gygax exerted proprietary control over the Paladin class, refusing, despite the urging of many gamers, to make the anti-Paladin an official character class. For years the anti-Paladin had been the most popular unofficial class additions to the game, with a particularly good treatment being done in Dragon issue 39. However, Gygax always argued with great conviction, albeit without particularly good arguments, the anti-Paladin unbalanced the game in most dreadful ways.

The Thief class added some much needed balance to the game, especially when dungeoneering. Dungeon settings were an earlier and popular setting for fantasy role playing, mostly because dungeons were confined and limited spaces made it much easier for the DM to map and handle character movement. DMs, with Gygax first among them, loved to load up their dungeons with various traps and tricks. As Magic Users gained levels they gained access to spells to counter traps and tricks like Knock and Invisibility. However, Magic Users were pretty limited at level one. This unfortunately made life very difficult for a Fighter/Cleric/Magic User party. This walking trap detection machine, however, had to be limited in fighting ability. For inspiration, Gygax's love for Fritz Leiber's Gray Mouser character comes into play. Like Gray Mouser, a D&D thief was weak in combat but strong in stealth and skills.
Greyhawk also brought a bit of Political Correctness to the game (long before political correctness was even a term and PC was still generally recognized by scant few as short for "player character"). The "Fighting Man" class became simply "Fighter", allowing the possibility that there existed women fighting men (like Red Sonja). Despite the name change "Fighting Man" persisted for a number of years. One can always recognize an ancient D&D gamer by his insistence on using "FM" ("Fighting Man") as a short form for the Fighter class.

Character classes themselves were changed in a number of ways. The original White Box D&D made little distinction between the fighting abilities of Fighters and Clerics. If Clerics could kick as much ass as Fighters and cast spells, why would anyone ever play a fighter? Greyhawk sought to create a greater separation between class combat abilities. For Fighters, Gygax introduced the rather odd 18+ strength bonus system. You were never Str 19. You were 18 (50). Higher strengths also gave combat bonuses under Greyhawk. Greyhawk also fleshed out the now-familiar bonuses for the other character abilities, like dexterity and intelligence. Classes now had their own Hit Dice progression: Fighters a d8, Clerics a d6, Magic Users and Thieves a d4. Additional spells and spell levels were added. Magic users had 9 spell levels. Clerics had 7 spell levels.

The use of additional Platonic solids for hit dice also allowed the game to vary weapon damage. Weapons for the first time had different damage ratings. A dagger now did 1d4. A sword did 1d8.

Greyhawk loosened up the rules on non-human player characters. The old White Box D&D rules were so restrictive one wonders why a person would ever want to play a dwarf or elf character. They were mostly restricted to Fighter. Non-human player characters could now advance to higher levels and the half-eleven race, a popular unofficial rule addition, was rubber stamped by Gygax in Greyhawk.

The savings throwless Magic Missile spell made its first appearance in Greyhawk. The Magic Missile spell and the "Do Dwarven women really have beards?" question were probably the two biggest debates found within the D&D community during the early days. They were the role player's "Go To Statement Considered Harmful" holy war.

Those in favor of Magic Missile as is liked it because

a) Even a low level Magic User could do some effective, ranged damage. After he threw his dagger, he was pretty much out of options. A Light spell wasn't going to really help the situation.

b) The spell when used by a higher level MU still did a respectable amount of damage, making a great "Hail Mary" play when all your Fireball and Lightening Bolt spells had been used.

c) Its short casting time and inerrant mark finding ability meant if one was confronted by an opposing magic user working on a much, much more powerful spell, a first level Magic Missile was just the trick to make that 12th level conjuror your bitch.

Those who didn't like the Magic Missile as is took issue with the last point. Since D&D rules stated that a magic user's spell was for not if he took damage while working on the spell, it did not seem right even a 1st level magic user could aim a single spell at a higher level wizard and, in effect, negate his spell by laying 2 points of damage on him. Giving the target a savings throw would remove what many saw as a loophole.

New monsters were added although this is arguably the weakest part of Greyhawk. Most of the new monsters were merely super sized variations of monsters detailed in the white box. For example, ogres now had a super sized ogre magi version. Cloud giants were trumped by Storm Giants. And should those Storm Giants get too big for their bridge-sized britches, there were Titans. A few well known monsters from mythology, missing from the white box, were added: the vampire, the harpy, and the golem.

New magic items also betray Gygax's love for traps and tricks over whack-an-orc adventuring. Greyhawk introduced the Rod of Lordly Might, the Cube of Force, the Bugs Bunny inspired Portable Hole, the Figurines of Wondrous Power, and the Deck of Many Things.

All in all, Greyhawk is probably the most important of the five supplements. Unlike Blackmoor, every concept introduced in Greyhawk made its way into AD&D.

See also for additional supplements:

II - Blackmoor, , III - Eldritch Wizardry, IV - Gods, Demi-Gods & Heroes, V - Swords & Spells