Just for those who don't know how open world PvP currently works in WoW, there is no such thing as a PvP server anymore since two expansions already and there is a PvP mode called War Mode for all servers instead.
Players with PvP mode turned on are phased in their own world, they can only see those with the PvP mode turned on so those with the PvP mode turned off are unable to see and interact with those with PvP mode on.
There is also a few incentives to having the PvP mode enabled.
What it gives when enabled:
  • Access to all PvP talents (PvP skills and abilities) in the outdoor world, regardless of being engaged in PvP combat or not.
  • +10% experience boost while leveling.
  • +10% World Quest (they're like FATE) reward bonuses at max level.
  • Ability to earn Conquest Points (PvP experience), which count towards a weekly progress bar for special gear rewards.
  • Completion of PvP achievements.
Turning the PvP mode on would require one to be in the main city (like Ul'dah, Limsa Lominsa and Gridania) and turning it off can be done anywhere provided one is out of combat.

Many players play with PvP mode turned off and enjoy the game just fine.