the 737 are split into three "generations". these are original, classic and next generation. the "original" models are the 737-100, 737-200/-200 advanced. the "classic" models are the 737-300, 737-400 and 737-500. the "next generation" models are the 737-600, 737-700/-700er, 737-800, and 737-900/-900er.