From WikiPedia

Skynet is a fictional artificial neural network-based conscious group mind and artificial general intelligence (see also superintelligence) system that features centrally in the Terminator franchise and serves as the franchise’s central antagonist.
Skynet is rarely depicted visually in any of the Terminator media. Skynet gained self-awareness after it had spread into millions of computer servers all across the world; realizing the extent of its abilities, its creators tried to deactivate it. In the interest of self-preservation, Skynet concluded that all of humanity would attempt to destroy it and impede its capability in safeguarding the world. Its operations are almost exclusively performed by servers, mobile devices, drones, military satellites, war-machines, androids and cyborgs (usually a terminator), and other computer systems. As a programming directive, Skynet’s manifestation is that of an overarching, global, artificial intelligence hierarchy (AI takeover), which seeks to exterminate the human race in order to fulfill the mandates of its original coding.