OK, who wants to be brave and tackle the differences in the MD-80 series? They include the MD-81, MD-82, MD-83, MD-87 and MD-88.

One major difference between the MD-80s and newer (MD-90, 717-200/MD-95) is the engine. MD-80s have smaller diameter PW engines, while the newer aircraft (as noted by Terenz) have IA and Rolls Royce engines with much larger diameter.

The 717-200/MD-95 project began as an update of the original DC-9 to serve the short-haul 100-passenger market. The MD-90 project was to update the MD-80 series aircraft with new engines, cockpit and interior.