The one before people / machines went back in time to alter the past. We (or at least, I) don't know what happened to Dyson on Judgement Day the first time around.
AFAIK, there was only one timeline, it was just a closed loop. Skynet was built using technology from the first Terminator (and presumably would never had been developed otherwise.) John Conner knew to send Kyle Reese back in time because his mother told him who came back in time to be his father (and even gave him a photo taken just after the events in T1.) The events of the first few movies play out as inevitabilities.
