CYCLING is divided into road and track events. Road cycling has been part of the Summer Paralympics since 1984, and track cycling since 1996. At the London 2012 Paralympics, 155 men and 70 women will be competing in 32 road cycling events at Brands Hatch in Kent (September 5 through 8) and 18 track events at the Velodrome in Olympic Park (August 30 through September 2). Cycling features tandem bicycles, for athletes with vision impairments; hand cycles, for those who pedal with their hands (track only); tricycles, for those who would have trouble balancing on two wheels (track only); and bicycles, some with modifications. Classifications are used to group athletes according to the severity of their impairment, with one classification for tandem, four for hand cycle, two for tricycle, and five for bicycle. Road cycling events include men's and women's individual road races and time trials for tandem bicycles, hand cycles, and bicycles; mixed road races and time trials for tricycles; and mixed team relay for hand cycles. Track cycling events include men's sprints for tandem bicycles; men's and women's individual pursuit and time trials for tandem bicycles and bicycles; and mixed team sprint for bicycles.
According to the description on the site of the International Paralympic Committee, "Cycling was first developed by visually impaired cyclists who competed using tandem bicycles, and it was introduced as a Paralympic Sport in Seoul in 1988. Today, in addition to visually impaired athletes, the sport includes those with cerebral palsy, amputations and other physical impairments. Athletes race on bicycles, tricycles, tandem or hand cycles based on their impairment."
You can read more about Paralympic Cycling on the site of the International Paralympic Committee.