1) Guild Wars is the original game, whose campaign is called "Prophecies" (you travel in the continent of Tyria, with 3 types of scenery);
2) Guild Wars:Factions is the second campaign released, where you can explore the continent of Cantha (Asian style);
3) Guild Wars:Nightfall is the 3rd campaign, set in the continent of Elona (African-based); this campaign enables you to use "heroes" (up to 3 henchmen whose skills and equipment you can control);
4) Guild Wars:Eye of the North is the 1st expansion (not a campaign, you need to have a campaign to use it) which extends the continent of Tyria (but you do not need to have the original game, only one of the 3 campaigns) with your level 20 character (and you get access to heroes too).
Each of these four gives you access to core elements of the Guild Wars game, plus elements specific to each campaign or expansion. You can of course combine them, and travel with characters from one to the other. One last thing: the game has two sides to it, PvE (Player vs Environment) where you play "against the computer" (possibly with some friends in groups) and the PvP (Player vs Player) where you play against other players. Each element gives you access to both, though only with the skills of the corresponding campaign. You can also buy PvP-edition where you can only play the PvP side of the game.
Bear something in mind: when you buy one of the games, you in fact buy an "access key" which gives access to the particular campaign or the first expansion to your account (you'll be asked to create one when you first run the program). You only need this, the rest can be either downloaded from the net (with the GW client, available here:
http://eu.guildwars.com/support/cate...nload_client/), or also installed on your hard disk via CDs (if you didn't buy the game from the the NCsoft website
http://www.plaync.com), each campaing putting around 1Go of data on your disk.
The "Platinum" edition bundles Guild Wars (original game) with Eye of the North. Each campaign also has a "Collector's edition" which provides a nice box with goodies and a few minor in-game stuff (one minipet, some equipment, one emote).