So to me the differance is simple an acquaintance is someone who you speak to be polite or in passing but a real friend is someone who you trust to your deepest secrets and you know they would never tell anyone. A friend is someone who you can talk to and never feel uncomfortable with, they would never judge you or activly seek to undermine your decisions. Unfortunatly for the majority of us will only ever meet maybe one person like this.

I'm not sure how one would become the other in my eyes but I think you just know if they are the sort of person who you can trust or not, of course everybody makes mistakes and get their fingers burnt but we live and learn I think.