Skynet is an artificial intelligence powered global defense system built by Cyberdyne Systems for the American Military. Skynet had access to all of America's stockpiles and defenses. When it became "self aware" Skynet deemed humanity a threat and not America's enemies. Skynet decided to exterminate humanity by sending a nuclear assault to Russia.
The Russian's quickly fired back at America and all of it's allies assisting in the mass extinction of human life. Skynet then used military prototype weapons such as HK's (hunter killers) and humanoid tanks the T-1; to round up the surviving humans. These survivors were used to build automated factories which in turn built more weapons to be used against humanity like Terminators.
The Terminators were created to more effectively oust humans from their hiding spots and were later giving coverings of living tissue to infiltrate human strong holds. These covering were originally very rubber like and easily detected by sight. As the covering became organic tissue that would bleed and react like human flesh; dogs were trained to literally sniff out terminators. All of the Terminators sent and received data from Skynet.
Humanity had learned to combat Skynet more effectively under the leadership of John Connor. John was turning the tide of the war so Skynet sent back a Terminator model T-800 to kill his mother before his birth. John sent back Kyle Reese who succeeded in ensuring John's existence and future. Skynet tried again with a super advanced T-1000, but was defeated by John Connor, Sarah Connor, and a reprogrammed T-800.
Skynet tried yet again with a T-X who had a sleeker more lethal frame than a T-800 and had a covering of liquid metal like what made the T-1000. The T-X was defeated and John lived to bring an end to Skynet.