A nun is a member of a religious community of women, typically living under solemn vows of poverty, chastity, and obedience and lives in the enclosure of a monastery or convent.  They live a life of prayer and contemplation.

A sister takes simple vows and live an active vocation of prayer and charitable works in areas such as education and healthcare.

POSTULANT NUNS

A prospective nuns first undergoes a period of testing the life for six months to two years called a postulancy.  She wears a white habit, or veil,  and undertakes the novitiate, a period (that lasts one to two years) of living the life of the religious institute without yet taking vows.  She then takes temporary vows which last for a period of one to three years.  After this she takes her “perpetual profession”  of permanent solemn vows.

CLOISTERED NUNS

Live with a wall separating them from the public although they may accept visitors.  They rarely leave their convent.

the number of nuns in the U.S. peaked in 1965 at 180,000 then plunged down to 56,000 in 2010.