In Roman Catholicism, a nun is a female monastic who has taken solemn vows (the male equivalent is a "monk"). Nuns are cloistered to the degree established by the rule of the religious institution they enter.
In the Roman Catholic tradition, there are a number of different orders of nuns each with its own charism or special character.
In general, when a woman enters a convent she first undergoes an initial period of testing the life, known as postulancy, for a period of six months to a year. If she, and the order, determine that she may have a vocation to the life, she receives the habit of the order (usually with some modification to distinguish her from professed nuns) and undertakes the novitiate, a period (that lasts one to two years) of living the life of a nun without yet taking vows. Upon completion of this period she may take her initial, temporary vows. Temporary vows last one to three years, typically, and will be professed for not less than three years and not more than six. Finally, she will petition to make her "perpetual profession", taking permanent, solemn vows.
In the various 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 the ideas of poverty and chastity). The "Poor Clares" (a Franciscan order) and those Dominican nuns who live a cloistered life take the three-fold vows of poverty, chastity and obedience. Most orders of nuns not listed here follow one of these two patterns, with some orders taking an additional vow related to the specific work or character of their order (e.g., to undertake a certain style of devotion, praying for a specific intention or purpose, ect.).
Cloistered nuns (e.g. Carmelites) observe "papal enclosure" rules and their monasteries typically have walls and grilles 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 have visitors in specially built parlors that allow them to meet with outsiders. They are usually self-sufficient, earning money by selling jams or candies or baked goods by mail order, or by making liturgical items (vestments, candles, bread for Holy Communion). They sometimes undertake contemplative ministries—that is a monastery of nuns is often associated with prayer for some particular good or supporting the missions of another order by prayer (for instance, the Maryknoll order includes a monastery of cloistered nuns who pray for the work of the missionary priests, brothers and religious sisters; the Sister Disciples of the Divine Master are cloistered nuns who pray in support of the religious sisters of the Daughters of Saint Paul in their media ministry; the Dominican nuns of Corpus Christi Monastery in the Bronx, N.Y., pray in support of the priests of the Archdiocese of New York).
A canoness is a nun who corresponds to the male equivalent, a canon. The origin and rules of monastic life are common to both. As with the canons, differences in the observance of rule gave rise to two types: canons regular and secular canons.
A nun who is elected to head her monastery is termed an abbess if the monastery is an abbey, a prioress if it is a priory, or more generically may be referred to as the Mother Superior and styled "Reverend Mother". The distinction between abbey and priory has to do with the terms used by a particular order or by the level of independence of the monastery. Technically, a convent is any home of a community of sisters—or, indeed, of priests and brothers, though this term is rarely used in the U.S. The term "monastery" is often used by communities within the Benedictine family, and "convent" (when referring to a cloister) is often used of the monasteries of certain other orders.