Changing the Guild Wars World

I keep reading how doing quests can change the guild wars world. That each player has influence in how guild wars saga will turn out. I don't understand. How does this work?

All zones outside of the towns/encampments are Instances just for your party. So anything you do can affect the instanced zone just for you. Other players have completely fresh instances when the go into that zone, so things you do don't effect them and things they do don't effect you, unless of course you are in the same party

Wow thats cool.

So what can happen in the area?

Which area? The town or the instance?

the instance.

You can kill or be killed, lol