The Cylons are a race of sentient machines and synthetic organic beings, whose name is derived from their initial development description as Cybernetic Lifeform Node. Created originally by mankind, they evolved and rebelled. Following a catastrophic war, they signed an armistice and left to found their own homeworld. Over the next forty years they developed humanoid models and began a second war, which almost wiped out both races.
The Cylons were developed from two separate concepts by Daniel Graystone and his daughter, Zoe. Daniel's company was under contract by Caprica's military to mass-produce a series of sophisticated robots which could assess a combat situation and plan countermeasures to new situations, something which at the time had never been done. The U-87 prototype was made functional thanks to the installation of Zoe's AI creation and was, eventually, copied into the bodies of thousands of robots, from military robots like the Centurions, to construction robots, to domestic service robots. The sophistication of the AI, based directly on her brain, resulted in thousands of sentient robots effectively living as slaves.
Eventually these Cylons began a general insurrection against the humans and ushered in a years-long long war against the Colonial military, constantly developing new weapons and technology to stay one step above the humans. In the twelfth years, the Cylons were contacted by a group of five humanoid Cylons who came from Earth, a planet which was destroyed when their own mechanical Cylons rebelled against them. The Cylons agreed to their plee to end the war in exchange for humanoid body manufacturing and resurrection technlogy.
In the next forty years, the Humanoid Cylons gained control of their emerging civilisation, with the Number Ones exiling their creators, later dubbed the "Final Five". Afraid of a Centurion revolt as befell the people of Earth, it was voted by representatives of the Seven models that chips be installed to prevent them thinking for themselves, thereby becoming slaves.
After forty years of isolation, the Cylons began a massive assault on the Cyrannus star system, wiping out all human colonies there and killing tens of billions within several hours. For the next year the Cylons conducted a search for survivors to ensure total eradication, but a revolution spearheaded by "Boomer" and "Caprica Six" led to the operation officially ending. A year later the Cylons made an attempt to co-exist with the humans on the newly-established New Caprica colony, but this failed due to the more fanatical models ensuring its failure through excessively violent responses to a resistance movement.
Following the abandoning of New Caprica, the Cylons began a mission to search for Earth, and interchanged between negotiating a truce with the humans and preparing ambushes to eradicate them. Unrest among the Humanoid Cylons continued, resulting in the entire line of Number Threes being boxed and new bodies prevented from growing. The Twos; Sixes and Eights launched a general insurrection against the other Cylons in response to the Ones' dictatorial actions, and most of them were wiped out. A damaged Baseship crewed by them allied with the Battlestar Galactica, and the repaired ship with human assistance was able to destroy the Resurrection Hub, ending the immortality of all Humanoid Cylons.