Cylons in the Original Series
In the original series, the Cylons were a race of robots that served a small number of alien masters (occasionally seen commanding Cylon Basestars). The exact reasons for the Cylon war against humanity were never clearly explained.
There were at least three models of Cylon seen during the series:
- Military models -- known as Centurions -- that served as footsoldiers, starfighter crew, and capital ship crew. They seemed to have very limited artificial intelligence.
- "Administrative" Cylons with more advanced AI that acted as advisors and managers.
- "Civilian" Cylons that performed non-military tasks on Cylon planets.
Cylons in the New Series
In the new series, Cylons were created by the Colonials to serve as soldiers in inter-colonial wars. They eventually turned on their masters and started a war to exterminate mankind.
The ruling variant of Cylons are a type of artificial human, virtually indistingishable from normal humans save for minor chemical alterations inside their bodies. They are stronger and faster than normal humans and have the notable ability to download their minds into cloned bodies during death.