The Terminator: Genisys trailer answers some questions but brings up a whole slew of new ones. We now know that another Terminator was sent back to kill the original Terminator that went after Sarah Connor in the original 1984 film. He succeeded, which changed the official "timeline" and basically eliminated the events of the first three films. (Side note: I watched Terminator: Salvation, but it was so bad I think I've blocked it permanently out of my brain, so I can't say whether or not that fourth film had anything to do with this timeline resetting business, and it really isn't worth my time or yours to find out.)
We meet Kyle Reese and John Connor in the future, as Reese (now played by Jai Courtney instead of Michael Biehn) volunteers to go back in time to protect Sarah, in the events that presumably transpired just before the first Terminator film began. When he arrives in 1984, though, Sarah Connor already knows who he is, and has apparently teamed up with a Terminator made to look like a 67-year-old piece of polished leather to try and prevent Judgment Day from ever happening.
As cynical as I am about all sequels and even franchises, these days, it's hard not to get excited when Arnold shows up and that brooding, industrial beat kicks in. (Seriously, if you haven't listened to it recently, it's one of the most bizarre film scores of all time.)