Most people use the term nuns to refer to both nuns and sisters, but there are some significant differences. Nuns’ lives are spent in prayer and work within their convent or monastery. Sisters are more active in the world, engaging in many different kinds of work, most often for people who are in great need. Sisters call these works “ministries.”
Both nuns and sisters are called women religious. What they have in common is that all women religious take vows to God, live in community, give themselves entirely to God for life and live in the spirit of the person/persons who founded their particular religious order.
When the Sisters of Mercy were founded by Catherine McAuley in 1831, most religious orders lived as nuns. They stayed in their convents and were seldom seen outside them. So when the Sisters of Mercy came on the scene in Dublin, Ireland, walking around looking for those in need, visiting the sick and poor people in their homes and visiting hospitals, people didn’t know what to think of them. So they called them “walking nuns.” Technically speaking, the Sisters of Mercy are sisters; that is, an apostolic community of women who combine a life of prayer with a life of active ministry. And we are still walking.