NJS is Nihon Jitensha Sinkokai or the Japan Bicycle Association. NJS exists to promote the bicycling industry in Japan. NJS is also tasked with regulating Japanese track bike racing, or Keirin. For those who believe the UCI's bicycle design requirements are backwards, Japanese Keirin rules -- established in 1957 -- are downright draconian. NJS specifies weight, frame geometry and material, the number of spokes on your wheels, and every component must have the NJS stamp of approval.
Because the NJS promotes the Japanese bicycle industry, non-Japanese manufacturers have found participation in the lucrative Keirin market very difficult. The list of approved component and frame suppliers consists almost exclusively of Japanese companies.
While the NJS cachet is not necessararily a mark of quality, there's a certain distinction to owning this equipment because the NJS mark is so exclusive. It's a bit like owning a numbered limited production run of a piece of art.