The following are extracts pertaining to Cylons, from the publication "Encyclopedia Galactica: From the Fleet Library aboard the Battlestar Galactica."
The Cylon androids, reptilian in form like their masters, continued the mission of destruction to which they had been assigned. Machines that they were, they swept through the galaxy even more ruthlessly than their creators. The directives of their programming crystallized into a single Edict of Extermination, that called for the destruction of all intelligent life forms in the Galaxy.
The Cylons have occasionally combined forces with beings like the Ovions, but few creatures are more useful to the Cylons alive than dead. Therefore, the number of members of the Cylon Alliance is likely to remain small. Cylons have no concept of friendship of loyalty, and are programmed to exterminate their living allies at the earliest convenient date.
Modern Cylons are basically humanform. The most common type is the Centurion, a heavily armored soldier capable of operating a Raider and piloting a Baseship, and also adaptable to planetary invasion and extermination. They are not of high intelligence, but can be cheaply mass-produced. A common Cylon strategy, therefore, is to overwhelm the enemy with large number of Centurions, many of whom will be destroyed, leaving a sufficient number of survivors to carry the day.
The most advanced Cylons are from the I-L Series. Both Lucifer, aide to Commander Baltar, and the Imperious Leader itself are of this type. I-L series Cylons have acute reasoning abilities, and can directly monitor electronic telemetry from up to fifty sources simultaneously. Much more than simple automatons, the I-L's at times exhibit humanlike drives for power. Human cylonologists speculate that this ambition response was deliberately programmed into the I-L's to insure prompt replacement of incompetent leadership.
The first human-Cylon contact occurred a thousand yahren ago, when combined human forces came to the defense of their allies, the Hasaris. The humans were unprepared for the massive Cylon offensive, and their fleet was soon forced back to the outer defense perimeters of their own Colonies. This was the beginning of the thousand-yahren Great Cylon War.
The humans proved to be the most formidable foes the Cylons had ever faced. Lacking imagination to respond to human advances creatively, the Cylons adapted in the only way they could: by imitation. The Centurions were designed to resemble humans more and more, and their weapons were redesigned accordingly. Human weapons advances were quickly met by Cylon copies, so that the war remained in stalemate for hundreds of yahren. The Colonial Unification Movement and the military genius of Commander Cain had pushed the Cylons to the breaking point, when the treachery of Baltar led to the near-final destruction of humanity.
The first Imperious Leader engineered the overthrow of the reptilian Cylons. Today, of all Cylons, only the Imperious Leader retains reptilian form.
Baltar, himself, was betrayed by the Cylons, who attacked all twelve colonies equally. Furthermore, their goal was not the enslavement of the human race under the dictatorship of Baltar, but rather the elimination of every human being from the galaxy. It was too late then for Baltar to return to the humans. The Cylons gave Baltar a choice between helping them complete their mission against the humans or slow death under torture. Baltar cast his lot with the Cylons, and appears now to enjoy his new rank of Commander of a Cylon Baseship. Although Baltar is not an inspired strategist by human standards, his imagination and knowledge of human nature make him indispensable to the Cylon Empire.
The following are excerpts from the episode novelizations Tombs of Kobol and War of the Gods, which were used in the very first version of the Battlestar Galactica FAQ, dated May 25, 1994. They were replaced with other material in subsequent revisions of the document.
"The legend has it that [the Ninth Lord of Kobol] received communication from another exploration ship that gave precise clues to the location of Earth. He, unfortunately, was a sceptic and did not believe in the hope that Earth represented. He also did not believe his planet was dying. It was said that he arranged for the crew of that exploration ship to be killed, and that he kept the secret of Earth's location to himself. His secret would not have been discovered but for some documents he left behind, documents that were carried to Caprica and enshrined there in the Planetary Museum... It said that the last Lord of Kobol had carried the secret with him to his tomb."