Cybertron is the homeworld of the Transformers.
Cybertron was built millions of years ago by the Quintessons, who took a previously organic planetoid and built it into a factory world. They installed the supercomputer Vector Sigma into its center, which split off a counterpart known as the Oracle, prior to Cybertron's completion. When the factory went online, it began to produce two lines of robots- Consumer Goods and Military Hardware. Over time, the Quintessons endowed the robots with an artificial intelligence so that they could build themselves, but they treated the early Transformers as slaves, cruelly oppressing them. This led to the First Cybertronian War, which ended with Cybertron in the Transformers' possession.
Thus began the Golden Age of Cybertron, where the Transformers of both production lines developed culture and civilization. However, conflict arose between the former Consumer Goods- now Autobots- and the former Military Hardware- now Decepticons. As the Second and Third Cybertronian War ravaged the planet, Cybertron's golden hue and vast stores of resources- not to mention huge chunks of the world's surface- were ruined and destroyed. With Cybertron in need of resources and embroiled in constant warfare, some Transformers left their homeworld to colonize other places, such as Paratron. When Earth was brought to the Transformers' attention in 1984, both sides tried to send resources back to their respective sides (using vastly different methods to get them, of course). At one point, the Decepticons even went to the length of pulling Cybertron into Earth's orbit, in order to turn the human world into a rapidly crumbling source of massive power. (It was later returned to its normal location.)
In the race for resources and conquests, however, the Decepticons were more successful, and ultimately conquered Cybertron by 2005. In 2005, Cybertron was shaken by the attack of Unicron, who destroyed the planet's twin moons and devastated the Decepticon forces, allowing the Autobots to take over. (Unicron's head became the new "moon" to replace the ones lost.) The Decepticons were driven to Charr, but the Autobots' chance crash-landing on the planet Quintessa reminded the Quintessons of their lost factory world. Still stinging from their defeat, the Quintessons began their own efforts to conquer Cybertron, nearly succeeding on a few occasions before they were driven to deep space.
After the Hate Plague was stopped, Decepticon leader Galvatron enacted a complex plan, wherein a massive Decepticon force gathered from across the galaxy conquered Cybertron anew, then pushed it near to Earth. Galvatron planned to open the Plasma Energy Chamber, using it to destroy Cybertron and Earth by overloading the Earth's Sun. His plan was thwarted by the Nebulons and Spike Witwicky (receiving hints sent out by Alpha Trion to Optimus Prime), who closed the Chamber and channeled the excess plasma energy to Vector Sigma. Using the energy, Sigma restored Cybertron to a semblance of its Golden Age.
Hundreds of years later, the Autobots' descendants, the Maximals, had won the Great War against the Predacons (descendants of the Decepticons), and kept the peace on Cybertron under the terms of the Pax Cybertronia. The Predacons schemed to regain their rightful place as rulers, however.
Both factions' plans for the future were ruined by the return of Megatron from the prehistoric past, who released a virus to lock all Transformers into their alternate modes, and conquered the planet using his Vehicons. The mad Predacon stole all the sparks and kept them prisoner, while seeking a way to eliminate his organic alternate mode.
The Maximals under the command of Optimus Primal opposed him, being selected by the Oracle to restore the long-lost organic elements of Cybertron. Primal at first misinterpreted this mission as one of destroying all the mechanical, but realized his error when he nearly destroyed Cybertron. Megatron was finally defeated, with he and Primal plunging into Cybertron's revealed organic core. The entire planet was reformatted into a technorganic world, and all the Transformers' sparks restored.
Originally a barren planetoid, it became the willing prison of Primus, who trapped himself and his enemy Unicron within the massive hunks of rock in an effort to stop Unicron from destroying the universe. While the drifting Unicron reshaped his prison into a massive robot body, Primus had a different plan. He turned his prison into a Saturn-sized planet of metal and technology. Next, after a failure on his first attempt created bizarre savage creatures, Primus created a race of warriors- the Transformers- who would would one day finish the work he had begun and defeat Unicron. After allowing them to increase their numbers for a time via bio-morphic reproduction, Primus erased their memories of the ability and left them with the Creation Matrix. Satisfied, Primus appointed the Keeper to guard his core and went into a slumbering state.
Cybertron settled into orbit around Alpha Centauri, and the Transformers gradually developed civilization. However, they split into two diametrically opposed factions- the Autobots and the Decepticons. The two sides began fighting a massive civil war, which wreaked havoc on Cybertron to the point that it was actually torn from its orbit, drifting through the universe.
After 1000 years of fighting, Cybertron drifted into Earth's solar system, where it was endangered by the asteroid belt there. Optimus Prime and Megatron, along with their troops, were lost on Earth, following the Autobots' efforts to clear a path for Cybertron through the belt (and the Decepticons' attempt to ambush the Autobots after they did so).
In 1984, after Prime and Megatron's forces reawakened, Earth became a center of conflict as both sides tried to send resources back to the needy Cybertron. In 1990, Unicron was able to find his ancient enemy when Primus was accidentally reawakened, and attacked in 1991. During the battle, Primus was slain, and Cybertron was wracked with quakes and storms thereafter, making it seem that Cybertron would follow its creator into death. However, although the Autobots and Decepticons abandoned the planet, Optimus Prime (in the body of Hi-Q) and the Neo-Knights learned the disruptions were actually part of a rebirth process. After the battle on Klo, Prime led the Autobots to resettle their healed homeworld.
Cybertron was subject to another attack from Unicron in 2006, paralleling the similar attack in the U.S. animated canon.