"To defeat the inhuman evils they pit themselves against, monks draw upon inner reserves of Spirit, a resource that reflects their personal mastery and training. Spirit replenishes slowly. It is generated in two ways — by specific skills and attacks, and by the completion of combos. Monks who chain their attacks carefully can continue a near-endless barrage of Spirit-fueled strikes. Spirit is a valuable resource, but massive expenditures of this force allow a monk to call upon potent dodges and defensive moves, as well as monster-slaying finishing strikes. Thanks to monks’ physical perfection and martial talent, they are free to deliver as many empowered strikes as their Spirit pools will allow — without waiting for ability ‘cooldowns.’"— Game Guide(src)
Normally a Monk has 250 maximum Spirit. Spirit does not normally regenerate, but many skills and items may add a slow replenishment to it. Items allow Monk to add up to +9 (+12 with legendary items) Spirit per second and +45 (+59 with Legendary items) maximum Spirit. Skills also add those: Exalted Soul, for example, adds +50 maximum Spirit and +4 Spirit regeneration, Chant of Resonance gives +4 per second (as long as any Mantra is taken), The Guardian's Path increases all Spirit generation by 15%, Circular Breathing adds another +3 per second, and Kormac may increase generation by 10%. Air Ally both adds +4 Spirit per second and restores 100 Spirit when activated. Paragon levels may grant +50 more Spirit. Spirit Generation from Primary skills can also be improved by up to 50% by Band of the Rue Chambers, and by up to +14 by Infused with Light.
Shenlong's Spirit Set reduces the Spirit regeneration to negative 65 per second while active, but also greatly increases all damage done.
In combat, it is mainly generated by primary skills aka Spirit Generators. Each attack of these skills replenishes 12 to 14 Spirit, with many special properties increasing that amount. Epiphany and some special effects give the Monk almost infinite Spirit for a short time. Spirit Stones, Daibos, and Fist Weapons may also spawn with affixes that can either increase maximum Spirit (up to +45, +59 with Legendary items), regenerate a Monk's Spirit reserves (up to +9 per second, or +12 with Legendary items), or even restore their Life when they expend it (up to 1248 Life per point of Spirit spent, plus up to additional 429 with Transcendence).