In most other MMORPGs, all characters populate the same game world, meaning players will often encounter other human players while playing.
Unfortunately, certain players exhibit "griefing" behavior, such as purposely preying on weaker players, stealing kills, etc. To avoid this, ArenaNet designed Guild Wars differently. For PvP (human players fighting other human players) there are designated areas called arenas. The rest of the world is strictly for PvE (cooperative play). To ensure this, a new instance—a separate, parallel copy—of the area is created each time a party enters an explorable area, with each such instance containing the same NPCs and mobs, but no other human players besides the party that spawned in that instance.
Exceptions to the above are staging areas such as towns and outposts. At these places, you can group, trade and socialize with other human players in the same district.