We are asked this question a lot and now Brother Richard Hendrick provides a nice reflection on the vocations of Monk, Priest, Friar and Brother.
Over the years, and particularly recently, many people have asked me what the difference is between the terms above... Sometimes it can even cause confusion to those who are discerning a vocation... "You're a brother but you say Mass???" Hopefully the following will help!
Priest: a Priest receives the Sacrament of Holy Orders (instituted by Christ Himself at the Last Supper) by which he is ordained to offer the Sacraments, especially the Eucharist or Mass, and the Sacrament of Reconciliation or Confession, and also to minister the Word of God to the people. He takes a Vow of Obedience to his Bishop or Religious Superior if he belongs to an Order, and a promise of Celibacy if he has not already taken the Vow of Chastity. (This is in the Roman Rite... Married diocesan clergy are allowed under certain conditions in the Eastern Rites and in exceptional circumstances in the Western Rite). The Priest is called to be an Alter Christus, another Christ, in that as he steps into the celebration of the Sacraments Christ chooses him to become present through him to the world in those sacred actions.
This is nothing to do with his own personal holiness or worthiness but is a grace conferred at the moment of Ordination when the Bishop lays hands on his head and prays the prayer of consecration.
A monk (from the Greek Monachos root meaning solitary or alone) is a member of a Monastic religious Order such as the Benedictines, Cistercians, or Carthusians to name a few. This is the oldest form of Religious or Consecrated life for Men. It may be lived in a solitary or community form. He takes perpetual vows, sometimes called Solemn Vows, by which he professes the Evangelical Counsels of Chastity, Poverty and Obedience, (often under the form of "conversion of life"), and to this may be added other vows such as Stability, (that vows him to a particular monastery), or even of Silence.
He follows a Rule of Life that establishes the conditions of day to day life enabling the monk to dedicate himself to prayer. He may be ordained as a priest or not. His life is dedicated to Prayer first and then to manual work and study and sometimes works of charity too. The Superior is known as an Abbot or Prior. Traditionally to meet a Monk you go to them! Their spirituality is often based on the scriptural descriptions of the gathering of the first disciples in the Acts of the Apostles...
The Mendicant (begging) Orders of Friars are a development/reform of the monastic orders that took place in the 1200's beginning with St. Francis of Assisi. The Friars also take Solemn and Perpetual Vows of Chastity, Poverty and Obedience and follow a Rule of Life. The Franciscan, Dominican, Carmelite, Servite, Trinitarian and Augustinian Orders (and many others) belong to this group. The Friars are bound to the Order but not to particular communities and travel to preach and to be present to the needs of the people. They find a balance between contemplative and active life with the various groups finding their own level of balance depending on their tradition and the teaching of their founders. Like their Monastic forefathers they may be ordained or not. The important thing is Profession of Vows first and Ordination is seen as a secondary Vocation within the primary Vocation of Friar. The word Friar is an old English word simply meaning brother. They may be referred to as Father if they are ordained but in the Capuchin Franciscan tradition, returning to the primitive tradition of St. Francis and the first Friars, the only official title is Brother and all of the Brothers are equal based on Profession of Vows and not on Ordination. As we Capuchins often say, "All of the Fathers are Brothers, but only some of the Brothers are Fathers!" Got it?
St. Francis also rejected any title that implied power over someone and asked that the superiors would be called Guardians rather than Abbot or Prior.... A reminder that they were servants of the fraternity and were there to guard the "places" of the Friars so that they would be free for the work of prayer and ministry.
Religious Brothers: Brothers such as the Christian Brothers, De La Salle or Presentation Brothers are part of a movement of Religious Congregations of men and women that began in the late 1700's. They are dedicated to specific apostolate such as teaching, nursing, missionary work etc. They may take either simple or temporary vows renewed every few years or perpetual vows taken once, often with promises related to their apostolate. They would be classed as active or missional rather than contemplative in character though often have a very deep spirituality of work. In the male congregations ordination would not be usual though some practice ordination for the sake of the community, ie a brother may be ordained for sacramental ministry to the brothers themselves...
While the above descriptions give clear boundaries to the various institutions it should be understood that on an individual level there can be blurring in the way an individual group or order understand themselves; however the above are the basic major categories of male religious life in the Roman Catholic Church...