Terminator - character - was created in 1984.

It turned out that [spoiler alert] Sam Worthington's character was actually a Terminator robot disguised as a human. The other Terminator robots looked like normal Terminators, although a CGI Schwarzenegger did fight John Connor.

That would be the T-800: Cyberdine Systems Model #101 Terminator from "The Terminator" series.

Terminator Salvation is a sequel. It is the fourth movie (#4) in the Terminator series with the continuing story of John Connor.

But in any science fiction movie having time travel, the concept of prequel or sequel gets blurred.

The fictional character John Connor got famous for appearing in the Terminator films. He was the daughter of Sarah Connor and leader of the human resistance against the machines. He was one of the lead characters of Terminator 2.