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.