"member of one of the mendicant monastic orders of the Church," late 13c., frere, from Old French frere "brother, friar" (9c., Modern French frère), originally referring to the mendicant orders (Franciscans, Augustines, Dominicans, Carmelites), who reached England early 13c., from Latin frater "brother" (from PIE root *bhrater- "brother"). Also in general use, "brother, friend, comrade" (c. 1300).
By the word [friar] is meant a member of one of the mendicant orders, i.e. those living entirely on alms, especially the 'four orders' of Franciscans, Dominicans, Carmelites, & Augustinian Hermits. [Monk] is used sometimes of all male members of religious orders including friars, but properly excludes the mendicants. In the latter case the general distinction is that while the monk belongs essentially to his particular monastery, & his object is to make a good man of himself, the friar's sphere of work is outside, & his object is to do a good work among the people. [Fowler]