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The Cylons are the primary antagonists to the human race. They are fully robotic in nature. They are responsible for the destruction of the Twelve Colonies of Man and it is from this holocaust that the a rag-tag, fugitive fleet -- later called the "Galactican Fleet" -- runs from.
While it remains unknown what happened to Baltar (having likely been left on a habitable planet, as promised by Adama in "The Hand of God"), the Cylons still pursue Galactica and her Fleet (Galactica Discovers Earth, Part I) without the Galacticans' knowledge.
Doctor Zee, a prodigy child from the stars, informs Commander Adama that the reason the Cylons have left their Fleet alone is because they knew that the Galacticans would lead them to Earth (Galactica Discovers Earth, Part I). Up until the attack on the Delphi, the Cylons had not attacked "in a generation" (The Super Scouts, Part I). While the Fleet finally discovers Earth, any landing or approach of the planet would alert the Cylons to Earth's existence, and therefore, scouts are sent to the planet in order to prepare Earth against the formidable Cylon enemy. Although there is a dissenter who disagrees with Zee and Adama's plan on doing so and causes problems for the forces loyal to the laws of the Galactican Fleet ("Galactica Discovers Earth, Part II", "Galactica Discovers Earth, Part III", "Spaceball").
While the Colonials gradually improve Earth's advancements in technology, an attack on Agro Ships by a wave of Cylon Raiders leaves the Fleet without a food source. This event forces the Colonials to use the resources of Earth to provide foodstuffs (Space Croppers).
Over the past 30 years, the Cylons have created better machines and evolved into different forms. There are two types of machines that have come from this drive, as so far as the Colonials become aware as of October 31, 1980.
On October 31 1980, the Colonials discover a shocking discovery: the Cylons have created a Cylon android, with the external appearance and agility of a human. These types of Cylons also have the ability to mimic human emotions and actions, including laughter (The Night the Cylons Landed, Part I).
In addition to being an advanced form of Cylon, they contain an electronic weapon in their hand of destructive power and have the ability to persuade humans through extra-ordinary, unspecified means. Andromus demonstrates this in two separate occasions, on Wolfman Jack as they leave Arnie's apartment, and later when a cabbie asks for fare after being dropped off at the WQSL radio station (The Night the Cylons Landed, Part II).
The massive A-B Raider is an advanced form of Cylon Raider. It has a crew of six, consisting of two Cylon androids (Andromus and Andromidus) and four Centurions. Its exact offensive and defensive capabilities are unknown. It seems to possess an offensive range far greater than that of conventional Raiders (and Colonial Vipers) as well as some form of communication jamming.
It is first seen during the attack on the Delphi (The Super Scouts, Part I), but is not apparently known to the Galactican Fleet until the ship encounters Recon Probe Delta and is subsequently probed by the Viper crew. Since Viper firepower proves ineffectual to this craft, it is rammed by the recon probe. This itself forces one of these Raiders, containing the two advanced model Cylons, into Earth's gravitational pull ("The Night the Cylons Landed, Part I") which results in the first known encounter between the Thirteenth Tribe and the Cylons, although this encounter remains unknown to the Cylon Empire as the survivors of the crash are destroyed by Troy and Dillon (The Night the Cylons Landed, Part II).
Centurions seem to have abandoned their use of laser rifles as their mainstay weapon and have instead gone to laser pistols sometime in the 1960s ("The Night the Cylons Landed, Part I", "The Return of Starbuck").
The Imperious Leader appears to have changed from the Original Series. It no longer has the voice of Count Iblis (War of the Gods, Part II) and instead seems to be far more impatient than its predecessor of 30 years prior. Out of this impatience comes an attack on the Galactican Fleet's agro ships, which forces the Fleet to create an agricultural colony on Earth out of a farm owned by Hector Alonzo (Space Croppers).