The Wii took approximately 45 months to reach the "30 million units sold" milestone, making it the fastest-selling console of all time. The best-selling console of all time is Sony's Playstation 2, which took 60 months to reach the 30 million mark.
As of June 2010, Nintendo has sold nearly 74 million Wii consoles worldwide, while Sony has sold 140 million Playstation 2 consoles.
The Wii has edged out the competition for a number of reasons -- mainly its unique motion-sensor input system (the Wiimote), and because of its more universal appeal. The input system is especially key in the Wii's success -- it uses a motion-sensor bar and an infrared "Wiimote" to record user's movements and allow them to more freely interact with the games.
However, this is all about to change, as both Sony and Microsoft have announced motion-sensing input systems for the Playstation 3 and Xbox 360, respectively, this year.
Sony's controller, called the "Move," is similar to the Wiimote -- an infrared controller whose movements are read by a webcam. Microsoft's input system is more revolutionary. Microsoft's input system, dubbed "Kinect," uses a camera, microphone, depth sensor, and processor, and records users' movements, faces, and voices. So, instead of holding a controller and moving around, like on the Wii and Sony Move, the user is the controller.
The Wii's success is not only a function of its input system, however. The Wii also features a number of family- and group-friendly games (such as the bundled Wii Sports), which gives it a more universal appeal. The Wiimote input system does require less "typical" video game skill (i.e. controller manipulation), though, and so many different users are able to play Wii games.