Cybertron was a heavily urbanized planet that originally was within the Unknown Regions, but by 1,000 BBY had been discovered by the Galactic Republic. It was populated by highly intelligent and independent droids.
No one knew what type of organic beings first created the robotic inhabitants of Cybertron, but possible candidates included the Celestials and the Rakata.
Autobots vs. Decepticons
By 500 BBY, the droids of Cybertron were sharply divided into Autobots and Decepticons. The Autobots under Optimus Prime preached tolerance of alien, organic life while the Decepticons, under Megatron, the conquest of so-called "flesh creatures".
Clone Wars and the rise of the Galactic Empire
During the time of the rise of Palpatine, the Decepticons fought against both the Republic and the Separatists in the Clone Wars, while the Autobots fought against the Decepticons' efforts against the aliens. After the end of the Clone Wars and the rise of the Empire, a joint Imperial-Autobot effort saw the final defeat of the Decepticons by 15 BBY. Cybertron then entered into a period of prolonged isolation.