Who are the CYLONS and why did they kill (almost) everyone?
As the opening credits to Battlestar Galactica explain every week, the Cylons were machines created by humanity. They rebelled, as sentient machines tend to do. The resulting war ended in an armistice that lasted 40 years, during which the Cylons disappeared into their own territory and developed the ability to mimic human form down to the cellular level. There are 12 models, seven of which were mass-produced and revealed over the course of BSG's first two seasons. When the body of one of these Cylons dies, its consciousness is automatically downloaded into a new body. They infiltrated humanity and unleashed a cataclysmic attack that wiped out all but some 50,000 human souls. Um, yikes. Other Cylon models include centurions, i.e. soldiers; raiders, i.e. fighter jets; and hybrids, i.e. near-human-seeming figures who control the Cylon basestars, their largest military ships.