The AIRLINERS.NET Aircraft Data and History pages describe the different types pretty well:
Douglas DC-9-10/20/30
Douglas DC-9-40/50
McDonnell Douglas MD-81/82/83/88
McDonnell Douglas MD-87
McDonnell Douglas MD-90
Boeing 717
Basically, the DC-9, MD-80, MD-90, and B-717 are all part of the same family of aircraft.
DC-9-10: Original 70-seat DC-9, entered service 1965.
DC-9-30: Almost 15 ft. longer, with 105 seats, entered service 1967.
DC-9-20: -10's fuselage with the -30's more powerful engines and longer wingspan.
DC-9-15: A -10 with more fuel and higher weights.
DC-9-40: Over 6 ft. longer than the -30 and more powerful engine options, seating 125, entered service in '68.
DC-9-50: The largest DC-9, a further 8 ft. longer than the -40 with 139 seats, as well as a new cabin interior. Entered service in 1975.
The DC-9-80 was renamed MD-80 in 1983, and is a generic designation for the whole series. It is a stretched and improved devolopment of the DC-9.
MD-81: First customer delivery in 1980.
MD-82: More powerful engines.
MD-83: Extended range with extra fuel and more efficient engines.
MD-88: MD-83's engines with and EFIS (Electronic Flight Instrument System) and a new cabin interior. First flight in 1987.
MD-87: Over 17 feet shorter than the rest of the series, similar in length to the DC-9-30. Also featured an EFIS, and options including a HUD, and more efficient engines. It also features improvements to the late-production -80s, such as the newer tailcone. First flight in 1986.
MD-90-30: Longest aircraft in the entire family. 4.5 ft. stretch to fit 10 more seats, newer IAE V2500 turbofans, the EFIS flightdeck of the -88.
B-717: Originally designated as the MD-95 before the MDC/Boeing merger in 1997, Boeing relaunched it as the 717-200 in early '98 (The "717" was originally allocated to the military C-135/KC-135 family). Fuselage is 4¾ ft. longer than the DC-9-30, and uses the DC-9-34's wing. Also has a 6-LCD EFIS flightdeck, and a cabin interior similar to the MD-90. A shortened -100 and a stretched -300 have been studied.