There is a difference. When you are talking about the year as a whole, you say "an". So you count with the word "an". (If there is a "full" number, better use "an") "Année" is when you are talking about it's content. You'd say "bonne année" for example. Also use "année" if you're using position (like 1st, 2nd, 3rd...) "La première année est la plus difficile".

Just like : Jour - journée Matin -matinée Soir - soirée chemise - chemisier

There no difference, It's just the feminin and the masculin version of the same word.

An : Period of 12 months. Année : Quinze Années soi SONT écoulées