What is the difference between obligatory fasting and voluntary fasting, along with the Prophetic Hadeeth indicating the difference?

For obligatory fasting, the intention must be made from the night before, and it is not permissible to break it except for a legitimate Shar'i excuse. As for supererogatory (nafl) fasting, it is permissible to make the intention during the day before zawal (midday), and it is permissible to break it without an excuse.