The warrior monk of Golarion is a recent addition to the lives of the peoples of the Inner Sea region. The combination of religious devotion, self-perfection, and martial combat is an import from the far regions of Vudra and Tian Xia. Monks are perhaps the rarest class in Golarion, outnumbered even by the elite paladins. The monk combines the piety of the cleric, the martial training of the fighter, and the deft skills of a rogue.
Avistan and Garund, depite their fame for monastic traditions, have been slow to adapt to this new way of life and still produce very few warrior monks from a select group of schools. Each monk school represents a specific way of life—a set of principles, practices, and philosophies, that combine with a unique fighting style to distinguish a practicing monk from his peers.
Monks more commonly learn from wandering masters—those who have seceded from their school, or have been sent to spread their teachings to the outside world. Each master carries with them a retinue of loyal apprentices—potential monks that learn habits of body and mind from their elder.
Any race might be inclined to the life of a monk. The orders are so small that the chances of finding any given race are almost equal. The lawful races have a natural disposition to the demands of monastic life, whereas the more chaotic races such as orcs are not likely to find the life tolerable.
In Avistan and Garund, the most notable and numerous monastic schools can be found on the island nations of Hermea and Jalmeray. These are likely the only places in the Inner Sea where one can find more than a handful of monks in one place. The regions surrounding the Crown of the World, being a frequent route of crossing from Tian, also have their share of monks who often stop to take on apprentices. 
Irori, the god of perfection, is a willing patron of monastic traditions. Those who seek physical and mental perfection are often drawn to his church, and may find their way into a monastary in the process. Likewise, those drawn to the martial traditions find Irori a useful deity in their pursuits, and this in turn might lead them to pursue the art of the monk.