Visually very similar to the 400ERF but without the extra MTOW and fuel tanks (Visual difference: Same as 400ERF just no big wheels)ħ47-400M - this is the combi version, designed for KLM I believe but used by a few airlines. (Visual Difference: No windows, shorter hump, nose cargo door, and main level cargo door, and once again, larger wheels)ħ47-400F - this is the freighter version of the base model. (Visual Difference: Larger wheels I think is the only thing you can see)ħ47-400ERF - this is the same as the 400ER just for freight so it has the shorter hump. It is similar to the base model but with a higher MTOW and additional fuel tank in the tail.
(visual difference: No winglets)ħ47-400ER - this aircraft was designed to operate MEL-LAX almost exclusively for Qantas. As most airports charge based on MTOW, this aircraft was cheaper as a people hauler from Tokyo to Sapporo where it didn't need the range. (visual difference from base: no windows, and main level cargo door)ħ47-400D - this aircraft was designed specifically for the Japanese market, it is a base 747-400 with a reduced MTOW. Same MTOW, same fuel capacity, same everything just with cargo and no passengers.
As downscc mentioned, wikipedia is a pretty good place to start but here is what I know:ħ47-400 - this is the base model, standard max take-off weight (MTOW), standard seating anywhere from 300ish to over 400 depending on the airline and standard fuel capacity.ħ47-400BCF - this is a base model that was converted into a freighter.