Skynet is the main antagonist of the Terminator franchise. It serves as the supercomputer that sets the events of the Terminator films in motion. When Skynet becomes self-aware, it perceives humans as its enemies and launches nuclear missiles around the world. It then tries to wipe out all survivors using an army of machines called Terminators. Dr. Serena Kogan of Cyberdyne Systems knew Skynet was going to wipe out humanity, so she built Marcus Wright as an infiltrator to catch humans and bring them back to Skynet Central.
In Terminator: Salvation, it was portrayed by Helena Bonham Carter. In Terminator: Genisys, as the T-5000, it was portrayed by Matt Smith.