From what I understand a fishplate is a term from prototypical railway .they are actually fixed to the side of the rails and have some boot heads visible. Model railway fishplates that I've seen emulate this effect by having some sort of plate on the side surface of the rails. However, people often use the terms fishplate and railjoiners interchangeably so that can be confusing. Rail joiners are the plain old things that join rails at the bottom .

To me, it's nothing more than "Fishplates Join Identical Rails Together" and the difference is simply in the name the Manufacturer chooses.

They LOOK alike and WORK alike... i.e. Insert Tab "A" ( the end of one rail) into Slot "A" (one end of a Fishplate,) then Insert Tab "B" ( the end of the next rail) into Tab "B" (the other end of the same Fishplate) and your Track and electrical connection, conductivity and continuity is completed !!  Repeat for each Rail and each additional length of Track you may need to add.  

To me, it's nothing more than "Fishplates Join Identical Rails Together" and the difference is simply in the name the Manufacturer chooses.

They LOOK alike and WORK alike... i.e. Insert Tab "A" ( the end of one rail) into Slot "A" (one end of a Fishplate,) then Insert Tab "B" ( the end of the next rail) into Tab "B" (the other end of the same Fishplate) and your Track and electrical connection, conductivity and continuity is completed !!  Repeat for each Rail and each additional length of Track you may need to add.  

Only half of your question has been answered so far - and even then no-one said the obvious! In model railway terms. fishplates and railjoiners are the same thing. The only difference I can think of may be that 'fishplates' might have the prototypical bolt heads pressed into them.

As for the difference between Code 100 and Code 75 rail, it refers to the difference in height of the rail head above the sleepers. Code 75 produces a rail profile that looks closer to the prototypical profile. Code 100 was originally produced to allow the earlier overscale wheel profiles to run comfortably. More modern wheel profiles are closer (albeit still over) to the prototypical profile and therefore can handle a finer scale track. Code 75 and Code 100 will co-exist quite happily - many people use Code 75 for the 'visible' portion of a layout and the cheaper Code 100 for the 'unseen' fiddle yards.