Can someone please explain the difference. I know the top sounds much better but won't most people see them as the same?

When you change the entire Exaust system it is not Fartcan.

Cat back: https://www.youtube.com/watch?v=4_wzBeHVfck
-Haven’t heard the latter “term” used at all

The reason a Subaru has a large diameter exhausted even stock is because of the flow and design of the turbo system

Fart can the large back section of the exhaust (shaped like an empty can) is empty causing the disgusting sound.
A cat back is a modification done to typically turbo charged cars to maximise the performance of the vehicle, has fins inside of it to make exhaust gasses less harmful.

Fart can refers to the style of muffler, cat back is the length of the exhaust system, from CATalytic converter to the back of the bumper. So in this picture we got cat-back fart can and just a fart can on stock piping.

Know the catback thing, I was more referring to appearance being the same.

They both are fart cans, literally it doesnt matter if it sounds good a fart can is sticking that metal can on the end of the exhaust

The sound bro

The fartcan is copying the catback. A fart can is like the nonames version of the original I guess