A friar is a member of one of the mendicant orders founded in the twelfth or thirteenth century. The term distinguishes the mendicants itinerant apostolic character, exercised broadly under the jurisdiction of a superior general, from the older monastic orders allegiance to a single monastery formalized by their vow of stability. Friars are different from monks in that they are called to the great evangelical counsels (vows of poverty, chastity, and obedience) in service to society, rather than through cloistered asceticism and devotion. Whereas monks live in a self-sufficient community, friars work among laypeople and are supported by donations or other charitable support. A friar may be in holy orders or be a brother. The most significant orders of friars are the Dominicans, Franciscans, Augustinians, and Carmelites.

The word "friar" is from fraire, which means "brother". The word arose with the creation of the mendicant orders in the late Middle Ages, most predominantly by Saint Francis of Assisi and Saint Dominic. These "new religious" were no longer tied to monasteries and convents but went out among the people, to preach and to pray, to educate and to serve the sick. A priest who is part of a mendicant religious order is also a friar; a priest who is part of a conventual/monastic community (contemplative/stationary) is also a monk. However, monks and friars need not be priests.

The duties of a friar include preaching, educating, and serving the sick. They work among laypeople and are supported by donations or other charitable support. The main orders of Catholic friars, the Dominican and Franciscan orders, include men who have taken holy orders. Other friars feel called to do the work of God without taking holy orders. Many friars serve in pastoral roles, while others work in education, social ministry, administration, and manual trades.