Cybertron is the home planet of the Transformers and (usually) the body of their creator, Primus. Cybertron is (almost always) a shining metal, technological world; a planet of towering future cities without end and vast metallic plains, spiraling metal mountains and bottomless neon-lit chasms. Battling over control of Cybertron and its resources is frequently the motivation for, or origin of, the Transformers' epic wars.
Cybertron is orbited by three "near-Cybertron" objects: Moon Bases One and Two and the Trypticon Asteroid Belt Station, each on opposite hemispheres of the planet (presumably the Iaconian and Kaonian sides). Beyond the moons and asteroid belt floats the last of the great space bridges, the long-abandoned interstellar transportation system that once connected Cybertron to its colonies Velocitron, Gigantion and the Hub elsewhere in the galaxy.
Unicron attacked this Cybertron early on in its history, but was defeated by the Thirteen. After their squabbles escalated into murder, the Covenant of Primusdecreed only one Prime could rule at a time, with the thirteen members of the High Council. The Quintessons seized control but an Elite Guard named Sentinel led the revolution, and with the High Council and the Guilds, he led the Transformers to a Golden Age of expansion. As the space bridges became less reliable and contact with the colonies was lost, Sentinel and the Guilds instituted the caste system to slow their civilization's decline. The High Council and their Guards were in charge of law and order, but each state controlled its own militia.
But as the nameless miners resorted to gladiatorial combat as an outlet for their misery, a champion named Megatron rose to seize the title of Prime for himself. Unwittingly, his Iaconian ally was appointed Optimus Prime instead, and their anti-caste movement became a civil war for Cybertron's future. The AllSpark was lost to space, whole regions of the planet were laid waste, and the core of Cybertron was poisoned by Dark Energon, the remnants of Unicron's attack, prompting the Autobots to evacuate on the Ark. Megatron and his elite forces left on Trypticon too, now modified into the Nemesis, leaving Shockwave to deal with the Wreckers.