What's the difference between a stan, a fan, and a casual listener?

firstly there is never going to be a definitive answer to your question since there are no set standards...

I can only go by what I personally would consider the differences...

Stan - (go listen to Eminem's song where the word originated from and understand the lyrics) obviously it might not be that extreme...

on the opposite end is the casual fan being someone who might listen to their music once in a while (TT mostly - not much bsides unless promoted)

fan is the middle of two...you're not a stan but you know more about the group than a mere casual listener

for example I say that I'm a fan of Gidle - I post in the guild regularly and keep up to date with some of their activities but I don't stream their songs, I don't attack their antis or defend them or buy their music so middle of the three...