The Boeing 737 compares to the Airbus A320. All 737 models have a dorsal fin at the base of the tail that forms an angle where it meets the fuselage, while the Airbus is straight.
The 737 variants that are in service today are the 3rd generation, comprised of the -700, -800, and -900 models, and the 4th generation, called Max7, Max8, Max9, and Max10 (all temporarily grounded worldwide as of 2020). The older generations are rarely seen anymore. Nearly all of the current 737s have winglets, either the blended shape or the scimitar shape, to improve fuel economy by reducing aerodynamic drag.
737-700 models can be identified by a single emergency exit over the wing, while all others have two. The only significant difference between each model is length, with the -700 being the shortest. The 737 Max has larger engines with chevron-shaped exhaust outlets.
When viewed from the front, the 737’s engines are not fully circular but slightly flattened on the bottom to help with ground clearance.