According to St. Thomas Aquinas, a canon regular is essentially a religious cleric, or, as the same doctor aptly expresses it: "The Order of Canons Regular is necessarily constituted by religious clerics, because they are essentially destined to those works which relate to the Divine mysteries, whereas it is not so with the monastic Orders." (II-II:189:8 ad 2um, and II-II:184:8). We have then here what constitutes a canon regular and what distinguishes him from a monk. The clerical state is essential to the Order of Canons Regular, whereas it is only accidental to the Monastic Order. Hence Erasmus, himself a canon regular, declared that the canons regular are a quid medium between the monks and the secular clergy.There is much more here at Newadvent.
To explain further the nature and distinctive spirit of the canonical order, we may say, with St. Augustine, that a canon regular professes two things, "sanctitatem et clericatum". He lives in community, he leads the life of a religious, he sings the praises of God by the daily recitation of the Divine Office in choir; but at the same time, at the bidding of his superiors, he is prepared to follow the example of the Apostles by preaching, teaching, and the administration of the sacraments, or by giving hospitality to pilgrims and travellers, and tending the sick.