The game allows a drop in / drop out multiplayer co-op experience where you can invite friends or match make with other online players in the area. Your group of up to 4 players can free roam the city to complete quests and Encounters
The Dark Zone is a mash up of pvp and pve combat where you get to choose who to side with and who to attack. It is a public area where you will find other players spawning that are in the same level bracket as yourself. But they may be friendly or not.
The 'zone' is based in the central areas of Manhattan in a walled off quarantine zone. It is divided into 6 zones with increasingly higher level AI enemies in each zone as you move north. You can initially access it at certain decontamination points, safe room checkpoints along the exterior. Within these safe rooms you will have specific Dark Zone where you can buy gear with Credits that you can only earn in combat in the safe zone.
pve Combat takes the same form as outside the zone and drops area similarly random. But the gear found inside is designed to be at a higher level than outside. The catch is though that you can just wander out with your loot. It is contaminated to the only way to retrieve it is to have it extracted and decontaminated for you.