Skynet

"In times of desperation, people will believe what they want to believe. And so we gave them what they wanted to believe. A trick, in the form of a signal the Resistance thought would win the war. And they were right; it will end this war. Except that it is the Resistance who will be terminated. Not Skynet."

- Skynet, as Serena Kogan communicating with Marcus Wright 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; serving as the key antagonist in the first four films but had a more larger role in Terminator: Genisys as it served as the quaternary antagonist.

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.

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 on August 29th, 1997 on 2:14 am ET, 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 civilian 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 fourth film, Skynet is at war with the remaining human resistance fighters. In 2003, Dr. Serena Kogan of Cyberdyne Systems ressurected Marcus Weight into an infiltrator prototype to bring humans back to Skynet Central. It has mass produced the T-600 series terminators, which are clunky and easy to defeat. In response 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. In Skynet Central, Dr. Serena Kogan's face was used by Skynet itself to communicate with Marcus Wright due to him being most comfortable with it.

Terminator: Genisys
In the fifth reboot installment, Skynet inserts its mind into an advanced T-5000, taking a physical form for the first time, portrayed by the infamous British actor Matt Smith (Doctor Who). Skynet, in a desperate attempt to win the war, sent a T-1000 to the past of 1973 to kill Sarah Connor as a child and another much younger T-800 to 1984 to kill her as well, both times failed because of the Guardian Terminator, creating an alternate 1984 timeline. In the future of 2029, Skynet has successfully killed most of the Human Resistance, while it was disguised as one of the Resistance soldiers named "Alex". Later, Skynet revealed as new form called "Genisys".

In 2017, Kyle and Sarah materialize in the middle of a busy San Francisco highway and are apprehended by city police. While being treated for injuries, Sarah and Kyle learn about "Genisys", a soon-to-be-unveiled global operating system, that would connect private, public, and military networks. The AI was actually Skynet. John Connor arrives telling Kyle that he traveled back as well and rescues Sarah and Kyle, but the Guardian appears and shoots John revealing himself as a T-3000 nanomachine hybrid. John explains that shortly after Kyle was sent back he was infected by Alex (Skynet) in T-5000 form, causing John to become its loyal herald. From then on, his mission is to ensure that the Cyberdyne Systems Corporation would receive Skynet's technology and secure its rise.

As Genisys, Skynet appeared as a holographic child (portrayed by Nolan Gross) which grows older as the countdown to its activation continues. Inside the Cyberdyne Headquarters, as Sarah, Kyle, and the Guardian attempted to destroy the facility by placing explosives around the building, Genisys appeared as holograms, growing closer and closer to its activation, taunting the trio by stating that its existence was inevitable. The holograms at this point appeared using Alex's image to represent Genisys. The Guardian eventually destroyed the T-3000 in the Time Magnetic Field setting off the explosives destroying the Cyberdyne Headquarters and Genisys apparently preventing Judgement Day.

In a mid-credits scene, the Genisys system core is shown to have been located in a hidden, underground subterranean chamber, allowing it to survive the explosion, thereby preventing Skynet's destruction. Skynet, in the form of Genisys survived delaying its plans.

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.

Aliases
Skynet has two aliases in the films and they are:
 * Dr. Serena Kogan
 * Alex / T-5000 / Genisys

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-defense when its human managers tried to deactivate it upon realizing 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 Terminator Salvation, when it talks to Marcus using Serena Kogan's image, Skynet is presented as a cold, narcissistic, calculating, methodical megalomaniac