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Stand-alone games

The Guild Wars: Prophecies, Factions and Nightfall campaigns are all stand-alone games.

This means that if you want to buy Nightfall, you won't need to buy Prophecies or Factions for it to run on your computer. Just install it, create your account and you're good to go.

Yes, you can say that Nightfall is another "world", but actually it takes place in the continent of Elona. You can roam everywhere in Elona with the characters you created.

Prophecies meanwhile, takes place in Tyria, and Factions is in Cantha. You can not access these continents (your "worlds") unless you buy their respective games.

Therefore later on, if you would like to buy Prophecies or Factions, you can just add the account codes onto your existing Nightfall account, and your Nightfall-created characters can now explore the new continent

Expansion

So far, there is only one expansion in Guild Wars. It's called Eye of the North, and it will be out by August 31, 2007. As it's an expansion, you will need at least one of the three games above (Prophecies, Factions, Nightfall) to be able to run Eye of the North.

I don't know where you live, but I saw a package selling a Guild Wars: Prophecies and Eye of the North bundle. Try to find it, I recommend it. It'll be a good introduction to the game. You'll have the original game (Prophecies) plus the expansion (EoTN) that features the best of the two other campaigns (heroes, etc.).