Instance

In most other MMORPGs all human players populate the same world. This means at any point in the world there is a chance of running into other human players. But these other humans are not always friendly, so players start competing, fighting eachother and stealing from eachother. The avoid the grief caused by this, ANet decided to do things a little different:

For PvP (human players fighting other human players) there are designated areas: Arenas

The rest of the world is strictly for PvE (cooperative play). To ensure this, a new "instance" (this means a separate, parallel copy) of an area is created each time a party enters the zone. Each instance of a zone is only populated by one party of humans and computer controlled foes and NPCs, but no other human players. This makes sure you don't run into other hostile humans.

Exceptions from the above are Cities, Outposts and Mission starting points. At these places you can meet other human players to form parties.