How the WildStar PVP world working
There will be PvP realms where you will be able to PvP freely with the opposite faction in the contested zones. In exile zones the exile player (Same for dominion in dominion zones) will have to flag for PvP for there to be any action, this is to avoid griefing of low level players. You are flagged in enemy zones. On PvE realms you will have to flag yourself in all zones to participate in PvP.
You will be able to attack faction capitals and other cities. Guards will be weaker on PvP realms than on PvE realms; however carbines have taken precautions to ensure that you will still be able to receive quests from quest givers etc.
You will gain experience from world PvP kills. There will be a diminishing effect on killing the same player over and over (ganking). You will not gain any experience for killing low level players.