The 'Classic' 737 is the -100 through -600, 100/200 using JT8D(the Football), 300 -600 using CFM 56 engines. The 'next generation' is -700 through -900 (which can seat more people than a 707-320 Intercontinental

) The 700 through 900 use a scaled down 767 wing design, if you look out the cabin window it ain't the 37 wing you're used to.
APS, Aviation Partners System winglets can be fitted to now, any model 737 since there is a winglet mod to fit early series wings, and pretty much every 37 rolling out of Renton has the winglets installed. (APS embarrassed the self anointed gods of the wing @ Boeing by getting to use the -800 prototype to prove the theories of their design even though Boeing POO-POOed them and said they'd never get near what the hoped for, the numbers were significantly better than expected and Boeing Wing Group sat down to a cold crow sandwich and chips

).
So.now we are @ series 3 of the 37 with the MAX version with bigger engine fans and redesigned pylons and nacelles and only 1 other commercial airframe has outsold the 737, the DC-3, and that took a World WAR, not bad for an airplane Boeing figured wouldn't sell more than 250-275 total back in 1967.
APS also makes the winglets fitted to 757's and is starting on 737 style winglets for AIRBUS to replace the curb feeler tips currently fitted.