With their distinctive attire, nuns are among the most recognisable members of the Catholic church. Like their male counterparts, monks, nuns swear vows to devote their lives to the church, living apart from the secular world in their own communities and adhering to strict rules that govern their behaviour. Over many centuries of monastic tradition, many different orders of nun have arisen, each with their own traditions and rules.

One of the earliest and most prominent monastic leaders was the 6th-century abbot Saint Benedict of Nursia, who established a monastic rule that was adopted by many monasteries within the early church. There are many different orders of Benedictine nuns, all following Benedict's rule. Benedictine nuns spend much of their time in prayer and contemplation, but the rule establishes a mixed type of monastic life in which work and community life are also important.

Nuns who belong to the Franciscan order are called "Poor Clares" after their founder, Saint Clare of Assisi. Like Franciscan monks, the Poor Clares swear vows of poverty, chastity and obedience. Poverty in particular is very important to the Poor Clares, who lead lives of great simplicity. Poor Clare communities tend to be small by comparison to the convents of other orders of nuns; this is in keeping with the teachings of Saint Clare.

Like the Franciscans, the Dominican order arose as part of a series of religious reform movements in the middle ages. Dominican nuns not only devote themselves to prayer and work but also spend a portion of their time in study; the order places high importance on education. Penance and silence are an important part of the Dominican spiritual life.

The numerous orders that follow the teachings of Saints Benedict, Dominic and Francis are far from the only orders of Catholic nuns. In addition, there are many other Catholic women who take religious vows but are not nuns. These women, called sisters, have less restrictive vows than nuns. Many sisters work as teachers, caregivers or charity workers. Like nuns, each organisation of sisters has its own purpose and rules. Because nuns are sometimes addressed as "sister," it can be hard to tell the difference between the two groups.