Mana regeneration

The condition of a game character gaining mana points over time is referred to as mana regeneration. Mana regeneration is made up of two components: Combat regen and Spirit regen. Combat regen is listed in combat under the Spells category. Spirit regen can be viewed by mousing over your Spirit stat in the character menu.

Out of combat, players will regenerate their full mana regen amount which is the combination of their Combat regen and Spirit regen.

In combat, players will normally only regenerate their Combat Regen value, however healer specs have access to the Meditation passive which allows 50% of their Spirit Regen to continue in combat.

Mana regeneration takes place once per tick, but it's measured in MP5 (Mana Per 5 seconds), because that's the unit used in the tooltips.

Spirit regen
Spirit is the primary stat that regenerates mana, and is primarily useful for healers. The mana-regeneration formula is as follows.
 * Druid, Shaman, Monk, Priest, Paladin
 * SpiritRegen = 1.1287*SPI
 * Mage
 * SpiritRegen = .282175*SPI (Estimated)
 * Warlock
 * SpiritRegen = 0

Combat regen
Combat regen is made up of a base regen value and any bonus in-combat regen you might receive (such as Meditation). The base regen value is 5% of your base mana pool per 5 seconds.

Other
Many talents and spells provide alternate means of regaining mana such as Rapture and Shadowfiend.

Other sources of mana include consumable items (such as Mythical Mana Potions), item enchantments (such as Darkglow Embroidery), and items (such as Fall of Mortality).