A nun is a celibate Catholic woman who lives her life, under formal vows, in service to God. Nuns usually work and live inside a convent and are part of a religious order. A nun will often wear a habit, a special covering for the head and body. It is usually black and white, sometimes brown or blue (depending upon the particular religious order), and signifies that one serves God.
Each order has its special mission. Some nuns work as teachers in Catholic schools or as nurses in hospitals. All nuns must take a vow of poverty, and any money they earn goes towards supporting their convent or order.
A woman who is a widow and not a virgin may become a nun, but a divorcee may not, unless approved by the Vatican. In Catholicism, women are not allowed to become priests, but becoming a nun is considered a fulfilling and rewarding choice. Women interested in becoming nuns are encouraged to try living in a convent for a time so they might decide whether they would really like to serve God in that capacity.
A beginner nun is called a novice and is required to take vows of chastity and poverty. Leaving the sisterhood after taking vows is frowned upon, much like divorce, but has become common in the United States since the reforms of Vatican II.