From Diablo Wiki
Regeneration (or regen) refers a resource refilling at a steady rate over time. Many things "regen" in Diablo III, most notably player life and various player resources, often at a rate boosted by player equipment and/or skills.
- Life Regen: The most common use of the term is for "life regeneration" where a player's pool of hit points refills. There is no inherent life regeneration for any skill in Diablo 3; it must be enabled via skills or item bonuses. (Which are quite common.)
- Resource Regen: Most class resources have a default regen rate, which can usually be increased by item bonuses, or refilled in chunks via skills or health orbs.
- Resources with an inherent regen rate: The Demon Hunter's Hatred and Discipline, the Wizard's Arcane Power, the Witch Doctor's Mana, and the Crusader's Wrath. (All of these have an inherent regen rate that can be increased by skills and items.)
- Resources that do not regenerate inherently: the Monk's Spirit and the Barbarian's Fury. (Skills and item affixes can enable regen for both these resources, especially the Monk's.)
Life Regeneration in Reaper of Souls
Life regen is much more prominent in Reaper of Souls, due to the more tactical gameplay, larger character hit point pools, the removal of life steal, and numerous skills and new items granting hps regen. Quite high values can be obtained via item modifiers and the changes to some skills causing them to regen a % of a character's life per second, rather than just a flat figure
- See more references.