The post explaining the new system is quite lengthy, but here's the idea in a nutshell: Alliances will earn victory points by establishing and fortifying camps as well as attacking or assisting other alliances, and to the victors (the ones with the most points) go the spoils (special rewards). After a period of time, a new phase will begin and all points and camps will reset. The system, which is meant to evolve, will offer different rewards, different goals, and potentially different ways to earn victory in different phases. For an in-depth look at AvA, check out the devs' detailed guide.
[Thanks to Dengar for the tip!]