What is the difference between "narzeczona" and "dziewczyna"?
narzeczona = fiancee
dziewczyna = girlfriend
A girl becomes girlfriend, when she decides to date a boy. A girl becomes a fiancee after a formal engagement. In the past it was very official and was treated almost as seriously as a marriage itself, breaking the engagement was a dishonour. Currently it isn't as serious as in the past, but still engagement is a formal declaration that the pair wants to get married and actually started planning it. I hope that helps ;-)