The words sister and nun are used interchangeably in common speech, even among sisters themselves. Technically speaking, sister is a generic term that applies to any woman who takes vows in a religious order (including nuns). The distinction actually comes between nuns and apostolic women religious.
Nuns live, minister and pray within the enclosure of a monastery and take public, solemn, perpetual vows.
Sisters live an active vocation of both prayer and service, helping people in the society. Sisters take simple vows that can be can be temporary or perpetual.