The Volturi Coven act as the unofficial royalty in the world of vampires, and are an incredibly influential coven. They are also considered 'nighttime patrons of the arts' as, because of their inability to sleep, they study the arts at night. The Volturi act as guardians, keeping the secret society of vampires hidden from the human world as needed. They often send their agents to travel from Volterra to prevent overzealous covens from exposing vampires through mass eradication of every vampire (and any humans) present. The coven has existed for over 3000 years, and is the largest coven in existence, followed by the Cullen, Denali, Mexican, and other covens.
Throughout the centuries, the Volturi have established and enforced a number of laws that all vampires are expected to obey on pain of death. There aren't many of these laws, and they mainly concern the secrecy surrounding the existence of vampires. The laws are not written down; to write them would in itself be an infraction. Instead, the laws are passed by word of mouth from vampire to vampire. Many vampires are born after the laws were established, and every vampire was taught of these laws by their creator. Though they think of the laws as an accepted fact, they do not know that it is really the Volturi's scheme to maintain control of the vampire world. To this day, many vampires continue to obey the laws. Like the Cullens are obeying, which leads them to move approximately every 6 to 7 years to avoid suspicion in the human world.
The Volturi Coven was first established founded by Aro, Athenodora, Caius, Didyme, Marcus, and Sulpicia. The coven originated in Greece and proceeded to gather vampires with useful talents to build up their power, and eventually relocated to Volterra, Italy. When the coven became strong enough, the Volturi usurped control of the vampire world from the Romanian coven, the original ruling coven, and presented the laws of prohibiting vampires from being noticed by humans. After the Romanians were defeated in the century-long war, their next target was the Egyptians, with only two survivors from the demise, Amun and Kebi. Their power increased incredibly after recruiting Jane and Alec; two vampire twins with extremely powerful gifts, and subsequently made the coven untouchable. They presented the twins for the first time when the Romanian Coven tried to regain control and defeated the army easily with their powers.