In the distant future, Mankind has colonized space, with clusters of space colonies at each of the five Earth-Moon Lagrange points. Down on the Earth, the nations have come together to form the United Earth Sphere Alliance. This Alliance oppresses the colonies with its vast military might. The colonies wishing to be free, join together in a movement headed by the pacifist Heero Yuy. In the year After Colony 175, Yuy is shot dead by an assassin, forcing the colonies to search for other paths to peace. The assassination prompts five disaffected scientists from the Organization of the Zodiac, more commonly referred to as OZ, to turn rogue upon the completion of the mobile suit prototype Tallgeese.
The story of Gundam Wing begins in the year After Colony 195, with the start of "Operation Meteor": the scientists' plan for revenge against OZ. The operation involves five teenage boys, who have each been chosen and trained by each of the five scientists, then sent to Earth independently in extremely advanced mobile suits (one designed by each of the scientists) known as "Gundams" (called such because they are constructed from a rare and astonishingly durable material called Gundanium alloy, which can only be created in outer space). Each Gundam is sent from a different colony, and the pilots are initially unaware of each other's existence.
The series focuses primarily on the five Gundam pilots: Heero Yuy (an alias, not to be confused with the martyred pacifist), Duo Maxwell, Trowa Barton, Quatre Raberba Winner, and Chang Wufei. Their mission is to use their Gundams to attack OZ directly, in order to rid the Alliance of its weapons and free the colonies from its oppressive rule. The series also focuses on Relena Peacecraft, heir to the pacifist Sanc Kingdom, who starts off as a seemingly ordinary girl until she gets caught up in the conflict between OZ and the Gundams, becoming an important political ally to the Gundam pilots (particularly Heero) in the process.
-Taken from Wikipedia