In the Roman Catholic tradition, there are many religious institutes of nuns and sisters (the female equivalent of male monks or friars), each with its own charism or special character. Traditionally, nuns are members of enclosed religious orders and take solemn religious vows, while sisters do not live in the papal enclosure and formerly took vows called "simple vows".
In the branches of the Benedictine tradition, (Benedictines, Cistercians, Camaldolese, and Trappists, among others) nuns take vows of stability (that is, to remain a member of a single monastic community), obedience (to an abbess or prioress), and conversion of life (which includes poverty and celibacy).
In other traditions, such as the Poor Clares (the Franciscan Order) and the Dominican nuns, they take the threefold vows of poverty, chastity and obedience. These are known as the ‘evangelical counsels’ as opposed to ‘monastic vows’ proper. Some Orders taking an additional vow related to the specific work or character of their Order (for example, to undertake a certain style of devotion, praying for a specific intention or purpose).
Cloistered nuns (Carmelites, for example) observe "papal enclosure" rules, and their nunneries typically have walls separating the nuns from the outside world. The nuns rarely leave (except for medical necessity or occasionally for purposes related to their contemplative life) though they may receive visitors in specially built parlors, often with either a grille or half-wall separating the nuns from visitors. They are usually self-sufficient, earning money by selling jams, candies or baked goods by mail order, or by making liturgical items (such as vestments, candles, or hosts to be consecrated at Mass for Holy Communion).
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Nuns flickr photo by iamsch shared under a Creative Commons (BY-SA) license
"Nuns" flickr photo by iamsch https://flickr.com/photos/iamsch/14407168014 shared under a Creative Commons (BY-SA) license