Guild Wars is a MMORPG (Massively Multiplayer Online Role Playing Game) game. It is in the same genre as World of Warcraft (sorry guys for mentioning the name that shall not be mentioned!!

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Difference is Guild Wars is free, no monthly subscriptions. If you buy the game that is the last you will have to pay.
Gameplay is also very different. Graphics are so much better than the cartoony look and feel of WoW IMO.
You don't have to spend hours playing Guild Wars to achieve something where raids in WoW can take hours at a time.
Guild Wars has an instanced world (the map is unique to only you and your party being in it, there are areas where you can communicate and see people from all around the world though), but for me it is not such a big problem. I actually like it this way in some respects.
You also don't 'live' on any specific servers, meaning you can interact with anybody and everybody that is playing Guild Wars, no matter where they are.
Guild wars released last year April 2005. They released an additional 'chapter' in April this year, called Guild Wars Factions. It is labelled as an expansion pack (but it actually isn't really), you don't need the original Guild Wars to play Factions.
Nightfall is the third release, coming later this year.
In 2 weeks time they will be having a special preview event for Nightfall to all current players.
Guild Wars Factions has ranked #1 in North America and Europe.
Earlier this year they had a free for all preview event for Guild Wars Factions. Anybody could play if they received an invite from an existing player. They might just do the same for Nightfall.
I am sure I have missed other details, this is just what quickly came to mind.
edit: and of course a very stable gaming system and advanced streaming technology is plusses too
