Every time something is changed, it creates a new path leading to a different future. You can travel back to the past, but once you do, any changes you make mean going forward is different than where you originated from. So I'd say there is a whole spectrum of parallel timelines.
So, if I go back in time and decided to poison the water supply of early primates in Africa to ensure the human race never existed, I could do that, and I wouldn't disappear or cease to exist, as everything from human history in my original timeline had to happen for me to be there.. that timeline happened and is continuing without me from where I left.. however I could never go forward again back into that timeline, as it's inaccessible to me due to creating a new future.. it exists for everyone who is occupying it and saw me leave, but for me it will now never happen in the new world I've created/am occupying.
In this way, terminators and Kyle Reece can be sent back from a future that the Connor's can prevent from ever even happening (from their perspective).
"No fate."