Greyhawk is a fantasy medieval campaign setting that is intrinsically connected with Dungeons & Dragons as we know it today.
It is set on the planet Oerth and primarily focuses on the Flanaess (from the Flan, a people of this setting), the eastern part of the continent Oerik.
A central Greyhawk theme is conflict. Kingdoms rise and fall; borders and alliances are fluid. The region’s makeup is strongly influenced by the mass migration of different groups forced into exodus following an apocalypse-like series of catastrophes known as the Twin Cataclysms which occurred roughly 1000 years ago. The Twin Cataclysms were the climax of a protracted war between ancient Baklunish and Suelish empires; the vast Dry Steppes and Sea of Dust, separated from the Flanaess by the Crystalmist and Hellfurnace mountain ranges, serve as a reminder of these catastrophic events (lower left-hand side of the map below).

Greyhawk is a gritty setting. While some of the various conflicts can be viewed in purely “good” versus “evil” terms (consider Vecna and Iuz), most are simply different factions and groups competing to survive with people at the mercy of imperfect rulers.
How will the party fit in? Time will tell….
One quick note on this setting: this campaign will begin on 572 CY (Common Year). This predates most of first published Greyhawk setting materials by 4 years. If you’re diving into Greyhawk lore, take note that some events described may have not yet come to pass. If you have any questions about this, please feel free to ask!