From a totally high level it's a mission with various 'things' that have to be done, and there are Terminators doing the 'things' that they have to do. The two sides will have crossovers of interactivity and may or may not be after the opposite outcome.
I'm not going to put anything else on it as that will spoil the storyline.
Suffice to say that the movies are all about "Save the cheerleader... erm, I mean save John Connor." This isn't about that, it's about tertiary but still very important 'things'.
I hope that is sufficiently vague! *ahem*
Feyjarl