Hearing/appeal is if you are on the border/they are not sure or you do not get approved. Interview was to check what facts they had, make sure they were on the same page/that your application was all filled out correctly/enough, etc. If the interview had gone badly and you were not approved, you (or your lawyer) would then request a hearing which would be before a judge who would take all your information and all theirs and decide. If you did not like what they decided I think then you would ask for an appeal/2nd chance to go through the whole mess again with yet another judge/panel. Often people try the first time one their own but there's a lot of stuff that is needed and ins and outs of the system so something gets left out or isn't stated the way they want to hear it, etc. so they do not get approved so they then go get a lawyer (who often takes them because all the preliminary ground work has been done/forms filled out and just needs tweaking better) and get approved on the next go-round.