I'll try to give a fairly simple explanation:
Series 11/21 - Domestic version of the DC-8 and the first ones to be built
P&W turbojet engines.
Series 32/33 - Overwater versions of the 11/21 series, they could longer
routes and I think they had more powerfull P&W engines
Series 40 - Overwater version with RR
Conway engines. The diference to yr.
eye was that the front part of the RR
Conway engines had a
smaller air intake than the P&W engine.
Series 50 - Same dimension that the other early series of the DC-8 but with
/F - All cargo version of the 50 series without windows.
Series 61 - Streched DC-8 with turbofan engines like the 50 series.
Series 62 - Shorter version of the 61, but bigger than the 10/20/30/40/50's
with new nacelles in the engines and with the biggest range in
the DC-8 family
Series 62F/AF - All cargo or with side-cargo-door version.
Series 63 - Same size as the 61 but with same type of engines and nacelles of
Series 63PF - Special version built for Eastern with some features of the cargo
63 ones but built as a passenger airplane. In case of convertion
it was less expensive
Series 71 - A 61 version with new CFM's. This version was harder to convert
because of the engines it used before, they needed to make
changes in the wing structure.
Series 72 - A 62 series with CFM's
Series 73 - A 63 series with CFM's
Between the 71 and 73 it's really hard to see a difference outside, the only way is to check out the history of the plane and find out.
I hope this helps. I know that you can be more refined but my description is like a easy-reference book.