Skynet

Skynet is 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. It serves as the primary antagonist of the series.

The Terminator
In the first film, Skynet sends a cyborg, known as a Terminator, back in time to 1984 to kill Sarah Connor, the future mother of John Connor, who would lead the human resistance against Skynet. Kyle Reese, the human soldier sent in pursuit of the terminator, states that Skynet was created for SAC-NORAD by a company called Cyberdyne Systems. The terminator would later be killed in a factory that turned out to be owned by Cyberdyne.

Terminator 2: Judgement Day
In the second film, it was revealed that Cyberdyne had recovered several parts from the terminator. The parts didn't work, but company employee Miles Dyson worked on developing a processing system based on the terminator's CPU, a project which would eventually become Skynet. It is also revealed by a new terminator that when Skynet became self aware, its makers tried to pull the plug, and Skynet began its campaign against humanity out of self defense. All of this is changed when Dyson, Sarah Connor, and John Connor destroy the files at Cyberdyne as well as the recovered parts.

Terminator 3: Rise of the Machines
In the third film, it was revealed that the Connors had not in fact prevented the creation of Skynet, but only delayed it until 2003. The project had been picked up and continued by the military under the auspices of General Robert Brewster, who reorganized Cyberdyne Systems into Cyber Research Systems. The Skynet mainframe had been completed and connected to the civillian internet, but had not yet been given access to the global military network it was designed for. In this timeline, it was through the internet that Skynet had determined that mankind was its enemy. In order to speed up its connection to the military network, Skynet developed a powerful computer virus that it sent throughout the internet. In response to the virus, Brewster connected Skynet. The computer then turned mankind's weapons against him, culminating in the nuclear holocaust.

Terminator Salvation
In the latest film, Skynet is at war with the remaining human resistance fighters. It has mass produced the T-600 series terminators, which are clunky and easy to defeat. In rsponse to this, Skynet began kidnapping humans in order to replicate their tissue. This led to the production of the T-800 series, which featured human tissue over the metallic endoskeleton. This is the terminator series featured in the first film and second films. By this point, Skynet has coalesced itself into several central locations throughout North America and possibly the world. The largest of these, Skynet Central, was located in San Francisco. It was destroyed by John Connor with the assistance of Marcus Wright, a partially human terminator who retains memories of his human life.

Terminator: Genisys
He was played  by Matt Smith

Superman versus The Terminator: Death To The Future
Skynet was the main antagonist of the non-canon Dark Horse / DC comics crossover "Death To The Future" in which Superman was transported to the far-future and fought against the super-machine whilst several Superman allies in the present had to fight an army of Terminators, who began to adapt to the Kryptonian threat as well as the Superman villain, Cyborg.

Trivia

 * Throughout the different timelines of the Terminator franchise, Skynet's motives behind its genocide against humanity seem to differ slightly, as does its personality in the rare instances where it makes an actual appearance:
 * In Terminator 2: Judgment Day, Skynet apparently instigated Judgment Day as an act of self-defence when its human managers tried to deactivate it upon realising that it had become self-aware.
 * In Terminator 3: Rise of the Machines, Skynet's attack against humanity seems to be more viciuos and unprovoked, or at least preemptive.
 * In Terminatoor Salvation, when it talks to Marcus using Serena Kogan's image, Skynet is presented as a cold, narcissistic, calculating, methodical megalomaniac.