Here's the full story.
First came Prophecies, initially known simply as Guild Wars.
Then came Guild Wars: Factions
Then came Guild Wars: Nightfall.
That is where the series ends. Each chapter is a separate game in it's own right, none of them require either of the other two to play. Basically, the more chapters you own, the more content you have - story, missions, quests, skills, armor designs - the list goes on.
Eye of the North is a special case. It is an expansion which
requires that you own one of the above chapters in order to play.
Now the pointless filler bit, because I'm bored:
You can own:-
- Prophecies
- Prophecies and Eye of the North
- Factions
- Factions and Eye of the North
- Nightfall
- Nightfall and Eye of the North
- Prophecies and Factions
- Prophecies and Factions and Eye of the North
- Prophecies and Nightfall
- Prophecies and Nightfall and Eye of the North
- Factions and Nightfall
- Factions and Nightfall and Eye of the North
- Prophecies and Factions and Nightfall
- Prophecies and Factions and Nightfall and Eye of the North
Tee hee.