Go is a Chinese strategy game that is ~2500-4000 years old. A major part of the game's appeal lies in its elegant design. It has simple rules, making it easy to learn, but surprising depth and exciting gameplay captivate players of all skill levels. Unlike games that focus on capturing, Go emphasizes building territory -- surrounding empty parts of the board with your stones (pieces).
If you're interested in learning the basics of the game, please visit The Interactive Way to Go or download Falling in Love with Baduk, a free PDF e-book provided by the Korean Baduk Association (Baduk is the Korean name for go). For further reading, please visit Sensei's Library, a go wiki.