In the 1978 Galactica movie and series and the 1980 spin-off, the Cylons were created to serve the reptilian race of the Cylon Alliance. Somehow, just prior to the start of The Thousand Yahren War, the android Cylons made a deal with the mysterious Count Iblis (meaning "Satan" in the Arabic language) and destroyed their biological masters. Only the higher ranking Cylons retain the physical appearence of their creators.
In the 2003 miniseries and 2004 full-length series which "reimagined" Battlestar Galactica, the Cylons are fundamentally different. They were created by humans as robotic workers and soldiers; executive producer Ronald D. Moore has hinted that they were used in wars between the human Colonies. Eventually they gained sentience, rebelled and fought the humans in a major conflict that devastated both sides. An armistice was agreed, following which the Cylons disappeared for over forty years. They returned en masse, launching a sneak attack with nuclear weapons which eliminated most of the colonies (similar to the original Galactica). This is the starting point of the 2003 Battlestar Galactica miniseries.
In the 2003 miniseries and 2004 full-length series which "reimagined" Battlestar Galactica, the Cylons are fundamentally different. They were created by humans as robotic workers and soldiers; executive producer Ronald D. Moore has hinted that they were used in wars between the human Colonies. Eventually they gained sentience, rebelled and fought the humans in a major conflict that devastated both sides. An armistice was agreed, following which the Cylons disappeared for over forty years. They returned en masse, launching a sneak attack with nuclear weapons which eliminated most of the colonies (similar to the original Galactica). This is the starting point of the 2003 Battlestar Galactica miniseries.