A FO usually relies on external biological filtration only and usually containing larger fish
A FOWLR is somewhat a contradiction in terms (fish only is fish only) once you introduce the LR then you have a reef tank without corals or inverts- however Im guessing your question is "what is the difference in the type of tank"
Well, the live rock makes it different. The live rock introduces a natural, internal biological filtration system and depending on how much live rock you have - this internal filtration system can replace the common external filtration system. Which is why most modern reef aquariums do not need or use external biological filtration.
How many fish you have depends on the size of the tank and the size and temperament of the fish. I don't think there are rules of thumb that would say "this many inches of fish per gallon". You have to use your own judgment, and if your just starting out the recommendation of others, either people online or the LFS.
Basically the difference is the aesthetics of the tank. A FO is exactly that - Your focusing on only fish A reef or liverock tank, the fish are secondary and if you have corals they can go down on the list. In other words they simply accent the habitat, not be the dominating creature. Having too many fish in a reef tank isn't good for the eye (because the fish look crowded) and not good biologically because of the waste they produce.
I hope this at least answers part of your question.
Thanks Robert