The Cylons have nonverbal ways of sharing information but do not have a hive mind and are most often shown talking to one another. Even identical Cylons are separate individuals. Cylon society is made up of the seven known, mass-produced humanoid models. For a yet unrevealed reason, the final five Cylons are completely unknown to both the humans and the Cylons, and Cylons are forbidden from trying to discover their identities.

At the beginning of the miniseries, a Six copy is involved in an intense sexual affair with Dr. Gaius Baltar. Pretending to be an employee of a rival computer corporation, Six seduces Baltar while helping him with his work on the colonial defense system. Caprica-Six then reveals her true nature to Baltar, and informs him that the Cylons will use the computer secrets that he has given her to infiltrate the Colonial defense systems, disable the Colonial military and attack the Twelve Colonies. That day, the Cylons launch their attack and destroy most of humanity. Six uses her body to shield Baltar from a blast from the attack, saving his life and sacrificing hers.

In the episode "Downloaded", this copy is downloaded into a new body. This Six, nicknamed "Caprica Six" by fellow Cylons, is viewed as a hero amongst the Cylon civilization for her complete success in her mission to compromise the colonies' defenses. She retains her sentimentality and expresses some regret at her actions, as evidenced by her constant visions of Baltar. This "Phantom" Baltar plays a similar role to her that the virtual or 'inner' Six plays towards the real Baltar, as a critical counselor and manipulator. Caprica Six is enlisted to motivate the resurrected Galactica copy of Sharon Valerii (Boomer, not Athena) to move out of her apartment and reintegrate into Cylon society. However, in defiance of their superiors, both Caprica Six and Sharon opt instead to aid a human — to the extent that Caprica Six murders a fellow Cylon to save a human. Caprica Six and Sharon then begin preaching peace with the humans as the way of their God. This leads them to take over Cylon culture and to resume the hunt for humanity, leading them to the colony of New Caprica, which they take over in a bloodless coup facilitated by Baltar, who surrenders to Caprica Six as soon as the Cylons arrive.

After the Cylon occupation of New Caprica, Caprica Six alienates the other Cylons with her desires for peaceful coexistence with humans. Her reunion with the real Gaius Baltar, however, shatters her illusions about her former pawn, as Baltar impotently allows the Cylons to bully him into enacting their oppressive tyranny upon New Caprica.

The other Cylons insist Baltar must sign an execution order for 200 insurgents in response to two suicide bombings by the human resistance movement. Baltar initially refuses and Caprica Six alone objects strongly to her fellow Cylons desire for mortal retribution. These objections result in her being shot in the head by fellow Cylon Aaron Doral. Her death and an immediate threat to shoot Baltar force him to sign the order. Caprica Six is reborn and helps Baltar escape onto a Cylon Basestar when New Caprica is ultimately abandoned by its population.

After settling on the basestar she becomes involved in a menage a trois with Baltar and Number Three and explicitly states that she loves both of them. She becomes involved in rescuing an inexplicably sick Hera from the Basestar. She assists in Athena's download and witnesses the obvious bond between Athena and Hera. Caprica subsequently disables and then kills Boomer who threatens to throttle Hera despite Athena's anguished pleas. She leads Athena and child to a captured Raptor and returns with them to Galactica, where she is promptly imprisoned.

From Wikipedia