Monasticism

Monasticism is a religious practice where people "leave the secular world" and spend their lives involved in religious work and/or worship as monks and nuns. It's practiced by Roman Catholic and Eastern Orthodox Christians, and rarely by Protestants. Episcopalians are between Protestantism and Roman Catholicism and there are Episcopalian/Anglican religious orders.

Judaism and Islam have no monastic traditions, but some Eastern religions such as Hinduism and Buddhism have similar religious fraternal organizations.

Christianity
In Christianity there are contemplative orders and active orders. Contemplative orders believe their primary goal is prayer and religious contemplation. Active orders do outside work such as teaching or nursing.

Roman Catholic monks have been involved in heinous child sex abuse; one notorious example was Brother Robert Best in Australia, who committed horrific abuse against many boys and caused at least one suicide. The Christian Brothers protected Best, and let him access more victims. According to another source, Robert Best together with another notorious paedophile priest Father Gerald Risdale caused at least 26 suicides which may be the tip of an iceberg.

In D&D
In Dungeons and Dragons and some other role-playing games, the "monk" character class is based (loosely) on the Buddhist monks of the Shao Lin temple. They are martial artists with quasi-mystical powers, who can totally kill you with their chi. They suck next to pretty much everyone, especially clerics and druids. Better play them elsewhere.