Ravnica, the World City
A world covered entirely by a city, Ravnica truly came to be when 10 powerful groups came together and signed the Guildpact, a magical contract that gave each a series of duties to fulfill in return for control of the world.
The Azorius Senate are the lawmakers, who form the legislative arm of the Ravnican government.
The Boros Legion are Ravnica's army and police force, keeping the piece at any cost.
The Orzhov Syndicate control both the bank and the church, weilding faith in one hand, and wealth in the other.
The Selesnya Conclave feed most of the population, running the farms and orchards.
The Izzet League are the inventors and artificers, who handle power generation and material production for the world-city.
The Cult of Rakdos are the artists and entertainers of Ravnica, though the demon lord they worship loves nothing more than destruction.
The Golgari Swarm handle the waste management of the city, doing all the dirty little jobs that no one else will. Their rot farms also feed those who can't afford anything else, giving them considerable clout with the downtrodden.
The Simic Combine run the hospitals and work tirelessly to improve the body and mind of their patients - whether they agree to it or not.
House Dimir are the spies, assassins and rogues of the world, and the function of the guildpact means that most don't even know they exist.
Finally, the Gruul Clans were once charged with protecting and maintaining the wild spaces of Ravnica, but the other guilds conspired to develop the planet such that no "wild spaces" were left, leaving the clans to devolve into a collection of scattered, barbaric tribes, the most powerful of which want nothing more than to bring down the edifices that brought them low.
Recently the 9,999th anniversary of the signing of the guildpact was celebrated, and everyone is looking forward to the Decamillennial.