The Boeing 737 Next Generation, commonly abbreviated as 737NG, or 737 Next Gen, is a twin-engine narrow-body aircraft produced by Boeing Commercial Airplanes. Launched in 1993 as the third generation derivative of the Boeing 737, it has been produced since 1997. The 737NG is an upgrade of the 737 Classic (–300/–400/–500) series. Compared to the 737 Classic, it has a redesigned wing with a larger area, a wider wingspan, greater fuel capacity, and higher maximum takeoff weights (MTOW) and longer range. It has CFM International CFM56-7 series engines, a glass cockpit, and upgraded and redesigned interior configurations. The series includes four variants, the –600/–700/–800/–900, seating between 108 and 215 passengers. The 737NG's primary competition is the Airbus A320 family.

Boeing later introduced the 737-900, an even longer variant stretched to 138 ft 2 in (42.11 m). Because the −900 retains the same exit configuration of the -800, seating capacity is limited to 189, although aircraft equipped with a typical 2-class layout will seat approximately 177. The 737-900 also retains the MTOW and fuel capacity of the −800, trading range for payload. Alaska Airlines launched the 737-900 in 1997, the 737-900 made its first flight on August 3, 2000, and Alaska Airlines accepted the delivery on May 15, 2001. The type proved unpopular, with only 52 delivered, before being replaced by the improved 737-900ER.
(Source: Wikipedia)