Get your thinking tackle around this one.
The new Star Trek is not a sequel, prequel or anything so crass. They're just about allowing it to be called a reboot. Yep, despite featuring the crew of the original Enterprise in their salad days, it does fit exactly into what fans would call current canon.
Some time after Nemesis, old Spock gets wind of some time-travelling Terminator style tomfoolery in which the Romulans plot to go back in time and deal Young Kirk a lethal spike in order to craft a Kirkfree universe for them to rule. Good try, but Spock's a canny bugger who simply follows them back - straight into the youthful Enterprise - to put a stop to their upstart behaviour.
Depending on whether we're following Back To The Future rules or not, the resolution of the film could essentially void everything from the last 40-odd years of Star Trek history, setting up Spock as some kind of mentor figure and creating a whole new, alternative timeline. A little bit wanky, but possibly the breath of fresh air the series needs.