Sonic Riders Zero Gravity is a racing title for Playstation 2 and Nintendo Wii, in which characters from the Sonic universe use hoverboards, or other vehicles, which are referred to as Extreme Gear -- a special class of extreme-sports equipment which uses air as fuel.
This is the sequel to the first Sonic Riders. The main campaign allows you to play through the hero story (as Sonic and friends) and the Babylon story. (as Jet, Wave, and Storm) Each of the two campaign are relatively short, but afterwards there are many quick-fire challenges to complete, many vehicles to unlock, and a small number of courses to unlock as well.
The Wii version features online functionality in the form of ghost racing. Both the Wii and the PS2 versions allow for split-screen play for 2 players.
Sonic Riders Zero Gravity is a significant departure from the last Sonic Riders, this is because drifting and boosting are removed entirely and are replaced with gravity control and gravity dive, respectively. The result is that these basic actions of cornering and boosting are now more powerful, but take longer to perform, consume more fuel, and you have limited control when using them.
One aspect in which the two titles remain fundamentally alike is that they both emphasize shortcuts. Each course has a plethora of shortcuts to take advantage of, if you can find them. Although many shortcuts require only gravity control and thus can be used by any character/vehicle, some shortcuts don't become accessible to you unless you have the required attribute. The 3 attributes are Speed-Type (required for grinding), Flight-Type (required to fly), and Power-Type (required to destroy certain obstacles) -- Which attribute you have depends on which vehicle you're using. However, even with the right vehicle, you can't use its attributes in a race until you collect enough rings, meaning that certain shortcuts might typically be inaccessible until the 2nd or 3rd lap in a race.