A Clockwork Lim
Lieutenant Commander
Yes and no.
It is one continuous timeline -- that's the entire point of a timeline. But the events of that timeline alter each and every time someone or something is transported into the past.
The original Terminator was a clusterfuck to the "original" future. There had to have been no less than two previous jumps that were erased when they rewrote the future. The first was the creation of John Conner by whoever originally sent the original (unseen) version of Reese back in time. The second was when Conner sent Reese back in time to perserve that version of the timeline and insure his birth. Someone else had to start the cycle, however.
That's really what was going on in the original movie. It was less about saving Sarah -- that was already predetermined when Conner sent his father back. It was, however, all about insuring Conner was born by making sure his father was there to see it happen.
Every movie, comic, episode, book, etc. that occured since then has altered the timeline in one way or another, effectively creating a different version of the future each time. Sometimes it's something minor that's easy to reconcile, othertimes it's massive (such as in the original movie).
But despite the changes, it's technically all one timeline. There are no alternate simultaneously-existing futures. There's just one future that keeps getting rewritten to one degree or another.
It's fascinating that the future is still happening at all due to the sheer number of time jumps that have occured. It's pretty odd, too, as both factions are being incredibly stupid about the whole ordeal. Skynet, in particular, should be less concerned about stopping Conner from being born and more concerned with either creating a whole new world further back in time (such as creating a bionetic housing for itself and sending itself back to say 0 A.D. after having sent back numerous Terminators/workdrones to create a power supply, housing, factory, etc. for itself and then eradicating humanity when they have no way to defend themselves) while humanity should be more concerned about doing something similar but in reverse. Even creating stockpiles of weaponry and vehicles by repeatedly sending the same object(s) back in time would be a great way to help swing the tide.
But c'est la vie.
Anyone want to give me a Blockbuster gift card for Christmas? 
