What is a Cylon?
It depends on how and from what series you view it. To our left we have our typical "Toaster" Cylon Centurion from the original Battlestar Galactica. To our right we have a new and improved Cylon that takes the form of a human being from the Re-imagined Series.
True, the original Centurion might look intimidating with its chrome armor and red eye, but Number Five, as this model is called, doesn't seem too fearsome. In the Original Series, the Cylons were a near-extinct reptilian race that created robotic soldiers to fight the humans for a thousand years. In the Re-imagined Series, the Cylon robots were created by man for war and labor. The Cylons later rebelled, with a bloody war with the humans that lasted for years. Disappearing after an armistice, the Cylons reappear 40 years later and launch a devastating sneak attack. In both series, the remains of humanity are on the run.
It was easy for the Original Series characters to know the good guys and how to identify (and shoot) the bad guys. However, the Re-imagined Series, with their humanoid Cylons that live covertly within the Fleet, makes knowing friend from foe much more difficult. There are many suspects as to who is and is not a Cylon agent. In addition Cylon Spacecraft are living beings in their own right, and their basestars are controlled by a being called the hybrid.