A friar belongs to a mendicant order, and the difference between a friar and a monk is not a nicety but a difference of whole purpose.
Mobile, not enclosed
A monk is bound by the vow of stability to a single house and lives an enclosed life ordered to prayer, while a friar belongs to an order, Franciscan, Dominican, Carmelite or Augustinian, whose members are mobile, not tied to one monastery, and directed outward to preaching, teaching and the care of souls in the towns. The word means brother, and the friar lives by the mendicant ideal of poverty, historically supported by the labour and the alms of the communities he serves rather than by the endowed lands of a monastery, so the whole economy and orientation of the life differ from the monastic.
A thirteenth-century innovation
The friars were a deliberate innovation of the thirteenth century, an answer to the needs of the growing towns that the older rural monasticism did not meet, and their combination of poverty with an active urban ministry of preaching and study reshaped the medieval church. A friar enters by novitiate and profession into an order governed centrally, in contrast to the autonomous monastery, and he may or may not be ordained a priest. The distinction between the enclosed, stable monk and the mobile, preaching friar is one of the basic divisions of Western religious life, and confusing the two misses the reforming purpose the friars were created to serve.