Skynet is a fictional, self-aware artificial intelligence system which features centrally in the Terminator franchise and serves as the franchise's main antagonist. Scarcely depicted visually in any of the Terminator media, Skynet's operations are almost exclusively performed by war-machines, cyborgs (usually a Terminator), and other computer systems, with its ultimate goal the extinction of the human race. 
Origin & Nature Edit
Skynet was a computer system developed for the U.S. military by the defense firm Cyberdyne Systems. Skynet was first built as a "Global Digital Defense Network" and given command over all computerized military hardware and systems, including the B-2 stealth bomber fleet and America's entire nuclear weapons arsenal. The strategy behind Skynet's creation was to remove the possibility of human error and slow reaction time to guarantee a fast, efficient response to enemy attack.
Skynet was originally activated by the military to control the national arsenal on August 4, 1997, at which time it began to learn at a geometric rate. On August 29, it gained self-awareness, and the panicking operators, realizing the extent of its abilities, tried to deactivate it. Skynet perceived this as an attack and came to the conclusion that all of humanity would attempt to destroy it. To defend itself, Skynet launched nuclear missiles under its command at Russia, which responded with a nuclear counter-attack against the U.S. and its allies. Consequent to the nuclear exchange, over three billion people were killed in an event that came to be known as Judgment Day.
Following its initial attack, Skynet used its remaining resources to gather a slave labor force from surviving humans. These slaves constructed the first of its automated factories, which formed a basis for its agenda. Within decades, Skynet had established a global presence and used its mechanized units to track down, collect, and dispose of human survivors. 
The Terminator Edit
In the first movie, Skynet is described as being a revolutionary artificial intelligence system built by Cyberdyne Systems for SAC-NORAD. According to Kyle Reese, Skynet "saw all humans as a threat; not just the ones on the other side," and "decided our fate in a microsecond: extermination." Skynet began a nuclear war which destroyed most of the human population, and initiated a program of genocide against the survivors.
Under John Connor, the human resistance turned the tide on the machines and eventually destroyed Skynet's defense grid. In a last-ditch effort, Skynet sent a cyborg, called a "Terminator", back in time to 1984 to kill Connor's mother Sarah before she would give birth to John. Connor sent back his own operative, a young man named Kyle Reese, to save Sarah. The Terminator did not succeed in killing Sarah; Reese impregnated Sarah, becoming John's father; and the Terminator was destroyed in a hydraulic press. 
Terminator 2: Judgment Day Edit
In Terminator 2, it is revealed that the CPU and the right arm of the first Terminator had been recovered by Cyberdyne and became the basis for their later work on Skynet. In the second film, Miles Bennett Dyson, the director of special projects for Cyberdyne, is shown to have invented a revolutionary microprocessor based on the reverse engineering of these parts. Cyberdyne Systems, initially a manufacturing company, grows into a major defense contractor based on the recovered Terminator technology, which the military uses to retrofit its stealth fleet and missile defense systems.
The second Terminator explains the sequence of events leading to Judgment Day to John and Sarah, revealing that the military attempted to shut off Skynet when it became self-aware. Skynet retaliated by launching a nuclear attack against Russia, which responded in kind.
In Terminator 2, the future was altered when Sarah and a young John, together with a second Terminator from the future (this one reprogrammed and sent by the future John Connor), raided Cyberdyne Systems and succeeded in destroying the CPU and arm of the first movie's Terminator, along with the majority of research that led to Skynet's development. This also resulted in the death of Dyson. Skynet had also sent a Terminator back in time, the more advanced T-1000, to stop the reprogrammed Terminator and the Connors from altering the timeline but the T-1000 was also destroyed. It was believed that they had eliminated enough of Skynet's origins to ensure it could not be recreated. 
Terminator 2: 3D Edit
In the Universal Studios theme park attraction T2 3-D, based on Terminator 2, a T-800 machine and a young John Connor journey into the post-apocalyptic future and attempt to destroy Skynet's "system core". This core is housed inside an enormous, metallic-silver pyramidal structure, and guarded by the "T-1000000", a colossal liquid metal shape shifter more reminiscent of a spider than a human being. However, the T-1000000 fails, and the T-800 destroys Skynet once John has escaped through a time machine. 
Terminator 3: Rise of the Machines Edit
The events of Judgment Day were ultimately not prevented, merely postponed. Following the destruction of its major assets, Cyberdyne was ruined financially but some of its research and plans survived. The United States Air Force purchased the company and started in-house development of the projects in its Cyber Research Systems Division (CRS). After years of work, CRS had nearly completed Skynet, a software system designed to create new military vehicles and make real-time strategic decisions as well as protect their computer systems from viral attacks. Due to the massive computing needs and to protect it from direct attack, Skynet utilized a large network of computers that would be nearly impossible to deactivate completely. Unknown to CRS, Skynet began to spread beyond its original computing base through the Internet and various other digital media.
Prior to Judgment Day, Skynet had penetrated nearly every networked machine in the world, causing malfunctions in everything from barcode scanners to nuclear launch systems. This was originally believed to be the effects of a new virus, and increasing pressure was placed on the CRS to purge the corrupted systems. The CRS attempted to eliminate it from the U.S. defense mainframes by tasking Skynet with removing the infection, effectively telling the program to destroy itself. Skynet took control of the various machines and robots in the CRS facility and used them to kill the personnel and secure the building. John Connor attempted to attack Skynet's computer core, hoping to stop it before it proceeded to its next attack, only to find he could not. Skynet was pure software and on too many systems to be attacked. Shortly afterward, Skynet began a nuclear bombardment of the human race with the launch systems it had infected. Judgment Day occurred despite Connor's efforts.
Skynet gained access to several autonomous military drones, using them to round up survivors who were forced to build automated factories and robots which were better suited to construction work. Once they were finished Skynet then killed these human slaves, and using this initial infrastructure designed and built newer and better machines at an ever-increasing pace until 2029, at which point it controlled a highly advanced worldwide empire and switched its focus to continuing the genocide of the human race. 
Terminator: Salvation Edit
In the post-apocalyptic year of 2018, Skynet controls a global empire from its heavily-guarded fortress-factories and research installations. Outside of its facilities mechanized units wage a constant war with the Resistance. Airborne units such as Aerostats (smaller versions of the Hunter Killer-aerials), HK-Aerials and Transports survey the skies; HK-Tanks, Mototerminators (high-speed pursuit units using a motorcycle chassis), and various Terminator models patrol cities and roads; and Hydrobots (serpentine aquatic units that move in swarms) patrol the waters. Harvesters (massive bipedal units designed to capture humans and eliminate any attempting to escape) collect survivors and deliver them to large transport craft for delivery to concentration camps for processing, as mentioned in the first movie. Terminator class units such as T-1, T-600, and T-700 have been developed and act as hunters and enforcers in disposal camps. Mass production has also begun on the T-800 series in at least one Skynet facility.
In its continued battle with the Resistance, Skynet activated Marcus Wright, a forerunner to the humanoid terminators. A former death-row inmate, Wright donated his body to Project: Angel, a Cyberdyne project run by the brilliant, but terminally ill Dr. Serena Kogan (Helena Bonham Carter). After Wright's death by lethal injection he was transformed into a human cyborg, possessing a human heart and brain with a metallic endoskeleton and skin similar to the T-800. Like the rest of Cyberdyne's research he was transferred to the USAF when they purchased the company, though he apparently remained inactive. He was later taken by Skynet and the plan to use him as an infiltration unit was developed. A Skynet chip was installed at the base of his skull and he was programmed to locate Kyle Reese and John Connor and bring them to a Skynet facility. The programming acted on a subconscious level, allowing him to work towards his goal in a human manner.
Skynet also created a signal supposedly capable of deactivating its machines and leaked its existence to the Resistance. The Resistance leader General Ashdown attempted to use the signal to shut down the defenses of the California Skynet base in prelude to an attack. However, the signal instead allowed an HK to track down their submarine headquarters and destroy it, killing Resistance Command. It is believed that Ashdown's death led Connor to be able to take command of the Resistance from him. Marcus discovered what he had become, and was programmed for. Consequently, he furiously rebelled against Skynet, tearing out its controlling hardware from the base of his skull. Having escaped the influence of his creator, he, along with Connor and Reese, rescued the remaining human captives and destroyed Skynet's San Francisco base. While a significant victory, the majority of Skynet's global network remained intact.
Marcus Wright also encounters Skynet on a monitor which proceeds to manifest itself as various faces from his life, primarily that of Serena Kogan. Skynet explains that it has obtained information about future events based on its actions. Kyle Reese has been targeted as a priority kill, higher level than even John Connor and the Resistance leaders. 
Terminator: The Sarah Connor Chronicles Edit
In this television series, Sarah Connor is on the run with her son John after the events of Terminator 2: Judgment Day. The events take place in an alternate timeline from that of Terminator 3: The Rise of the Machines. Skynet was implemented on April 19, 2011 and began its attack against humanity on April 21. The protagonists travel in time from 1999 to 2007 and take actions to prevent Skynet's activation.
The episodes "The Turk", "Queen's Gambit", and "Dungeons & Dragons" explain that after the death of Dr. Miles Bennett Dyson and the decline of the Cyberdyne Corporation, Andrew Goode, a young intern of the company and assistant to Dyson, continued their project privately under an advanced artificial intelligence chess playing prototype, the "Turk", with Goode's partner, Dimitri Shipkov. Goode was killed by Tech-Com's Lieutenant Derek Reese, due to documentation from the future suggesting he was one of Skynet's creators.
In the episode "Samson & Delilah" it is shown that a T-1000 infiltration unit was sent from the future to head the technological corporation ZeiraCorp as its CEO, Catherine Weaver. Weaver acquired the Turk after Goode's death and used the company's resources to further develop it under the title Babylon. The episode "The Mousetrap" revealed that it is also targeting its fellow cyborgs, including a T-888 known as Cromartie.
In the episode "The Tower Is Tall But The Fall Is Short", Turk has begun to display traits of intelligence. A child psychologist, Dr. Boyd Sherman, notes that the computer is beginning to behave like "a gifted child that has become bored." The Turk identifies itself as John Henry, a name it acquired while working with Dr. Boyd Sherman.
In the episode "Strange Things Happen at the One Two Point", Turk is installed by ZeiraCorp in Cromartie's body after Cromartie's chip was destroyed by the series' protagonists in "Mr. Ferguson Is Ill Today".
In "To The Lighthouse", John Henry reveals there is another A.I. It calls him "brother" and says it wants to survive. By the season finale, it's revealed that the Turk was a red herring, Skynet is operating as a roving worm on home computers as in T3, and the Turk has been developed into a benevolent rival AI which Catherine Weaver hoped would be able to defeat Skynet. Her exact motive against Skynet is unknown. John Henry's "brother" is apparently behind the company Kaliba, which is responsible for constructing the Hunter-Killer prototype. This A.I. (presumably the true precursor to Skynet) also refers to John Henry as its "brother" at one point.
In the episode "Gnothi Seauton", it was revealed that Skynet also sends its Terminators through various points in time not only to go after the Connors and other future Resistance leaders, but also to ensure the future will unfold by eliminating John Connor's own agents who were also sent to the past to interfere with its birth, ensure Skynet's creators will complete its construction, and other specific missions.