What is the difference between a nun and a sister?

In strict Catholic usage, a nun ordinarily belongs to a contemplative monastic community, while a sister ordinarily belongs to an active religious institute. Both are women religious consecrated to God through public vows.

What they share. Nuns and sisters respond to Christ by embracing the evangelical counsels of chastity, poverty, and obedience according to the rule of their community (CCC 915, 925). Their consecration is rooted in Baptism and ordered toward more complete service of God and the Church. Religious profession is not an eighth sacrament and does not confer Holy Orders. It is a public, stable form of consecrated life recognized by the Church (Canon 607).

What a nun is. A nun traditionally lives in a monastery devoted chiefly to prayer, contemplation, the liturgy, and work within the community. Poor Clares, Discalced Carmelite nuns, and many Benedictine women are examples. Nuns commonly observe some form of enclosure, which protects their contemplative calling and limits ordinary outside activity. Their apostolate is especially prayer, sacrifice, and witness to the life to come. Their hidden vocation is fruitful for the whole Church, like the prayer of Anna, who worshiped God with fasting and prayer (Lk 2:36-38).

What a sister is. A religious sister usually belongs to an apostolic institute whose members serve more directly in the world. Sisters may teach, nurse, run shelters, serve in parishes, work with immigrants, or evangelize. They still share community life and prayer, but their rule is shaped around an active mission. Examples include the Sisters of Mercy and the Missionaries of Charity. Their service expresses the Church's call to recognize and love Christ in those in need (Mt 25:35-40).

Why the words overlap. In ordinary conversation, people often call any woman religious a nun. Some communities also use titles according to their own history and customs, so the distinction is not always obvious from clothing or ministry. A consecrated virgin, a member of a secular institute, or a lay member of a spiritual association is not automatically a nun or religious sister. If you meet a woman religious, calling her Sister is normally respectful. You can also ask the name of her community and what its particular charism, or spiritual mission, is.