Ravnica is a metropolitan planet with little to no nature left on it's surface. The entire plane is governed by several ruling guilds; these guilds maintain order, peace and stability for the billions of inhabitants.
Ten thousand years ago, the plane of Ravnica boasted untold violence and brutality, with nine factions constantly waging war against one or more of the others. Realizing that this never-ending war would ultimately destroy everything, a council between all nine factions was held, where the leaders suggested establishing a living, breathing enchantment that would end the violence and ensure the survival of all the factions. Each of the nine factions signed the Guildpact, and over time, these factions became the nine guilds of Ravnica.
The power of the Guildpact is subtle; it prevents any guild from disturbing the balance among the nine by twisting circumstance and coincidence to nullify the disruptive guild's actions. While the Guildpact is credited with bringing relative peace and prosperity to the plane, sporadic clashes between guilds do still occur. Despite any conflicts between the guilds at a particular time though, the anniversary of the Guildpact's signing is celebrated every year with a days-long festival, where all the people lay down their weapons and make merry. This celebration is called The Festival of the Guildpact.
Not all of Ravnica's citizens are a member of a guild; the vast majority have no attachment, and are referred to as 'guildless'. While not technically any less then a guild member, there are often slights against the guildless, even within the Law.
However, the guilds are a required part of daily life, and their presence is felt everywhere on Ravnica. The guilds manage most major operations on the plane; from the creation and distribution of food to the maintenance of what little nature remains in the world. The guilds are briefly described below:
The Azorius Senate is the legislative and judicial body of Ravnica bureaucracy. They're a collection of knights and paladins, bolstered by mages capable of subduing entire riots in seconds without harming a soul. Their Guildmages are lawmakers and politicians. The Guild Leader is Grand Arbiter Augustin IV, a lawmage and judge of incredible power.
The Boros Legion is a standing army that protects the Guildpact and contains the League of Wojek, the official peacekeepers of the City of Ravnica. They're wizards hurl bolts of fire and lightning at any whom break the Laws of Ravnica. Their Guildmages are decorated soldiers and mage-knights. The Guild Leader is Razia, an Archangel of enormous power and skill. She is rumoured to be one of the most powerful individuals on Ravnica.
The Cult of Rakdos, considered a necessary evil by some, the thrill-killers provide the heavy labor force of the plane. Many of the Rakdos are idiotic and mindless servants of the demon Rakdos, but the more powerful members are deadly assassins and blood mages. Their Guildmages are crazed cultists and fierce warriors. The Guild Leader is a mighty demon lord named Rakdos, whom dwells deep beneath the surface of Rav.
The Golgari Swarm which manages food production and organic waste disposal, and dwell beneath the city in the near-lightless world of Old Rav. The law has a slightly looser grip deep beneath the city, so many people respect and/or fear the Golgari and their zombie minions. Their Guildmages are insect-worshipping necromancers and frenzied druids. The Guild Leader is Savra, a dark elf necromancer said to possess ancient secrets of undeath that no other necromancer knows.
The Gruul Clans have fallen from their former glory as the keepers of Ravnica’s wilds and now are nothing but a loose affiliation of berserker clans. Their Guildmages are reckless pyromancers and shamans. The Guild Leader is Borborygmos, a cyclops barbarian who uses his size and strength to attempt to unite the scattered clans.
The Izzet League responsible for the civil engineering works and new magical developments. Their Guildmages are mad scientists and brilliant inventors. The Guild Leader is Niv-Mizzet, an ancient and extremely intelligent Dragon Mage.
The Orzhov Syndicate that regulates trade and banking, as well as running the courts of Ravnica. The guild fronts itself as a church, but almost everyone knows they're true god is money. Their Guildmages are cunning lawyers and lawmages. The Guild Leader is a collection of ghosts and spirits known as the Ghost Council.
The Selesnya Conclave which promotes a peace-loving and nature-worshipping religion. They are bolstered by their own private army, the Ledev Guard. Their Guildmages are spiritual clerics and skilled rangers. The Guild Leader is a collection of Dryads known as The Chorus of the Conclave.
The Simic Combine which provides medical assistance and the biological research. Their Guildmages are doctors and experimenters. The Guild Leader is Momir Vig, a brilliant and cruel scientist with enough knowledge to write his own library locked away in his head. [/spoiler]