tut at not knowing of it lol.
Its an RPG game set in the land of Tyria. Over the course of the game you will travel throughout the entire land completing quests and missions either on your own, with computer AI (henchmen) or other people (The PvE (player v Element) side of the game). You can download the game client from the official site and buy a game account online, or buy a boxed version from most games places.
You can  also take part in PvP (player v player) battles teaming up with others (either randomly or with your friends / other guild members) to sort out the strongest (im a bit useless at PvP lol)
You can play as one of six primary professions; Ranger, Elementalist, Warrior, Monk, Necromancer, Mesmer. Early in the game you also take one of these professions as a secondary (basically you have one skill-set less than the primary version).
There's also a new GW release called Factions, which gives you access to the new land of Cantha, two new character slots and two new professions; the assassin and the ritualist.
If ya wanna know more, go to
www.guildwars.com (the official game site), or ask me and I'll try and answer :P
If ya do decide to give it a go - might see ya there