Mana is the currency of magic, and is spent each time you cast a spell in Dungeon Keeper 2. Mana is also used to keep imps alive, at a cost of 7 mana per imp per turn. Mana is generated from owned land (including Reinforced Walls) at a rate of 1 mana per tile, but your 5x5-tile Dungeon Heart generates an extra 5, for a total of 30 per turn. The maximum amount of mana that can be stored in the Dungeon Heart is 200,000, with a maximum base income of 500/turn. Mana vaults add 100 to the income cap and to the income. Some traps impose a continuous drain on your mana, as well as consuming a larger amount each time they are activated.
Vampires have the advantage of praying automatically, if there is nothing to research and there is room for them at an Unholy Temple. They will pray even to the exclusion of training, which is handy as you do not want them to train up to level 4.
Despite their relatively low mana output, a horde of Skeletons can be a good source of mana if you can get them past level 4 (or put them in the Unholy Temple before they train up to level 4), because they will literally stay there and pray forever until you make them do something else. You can lock 24 of them in a 5x5 room, all filled with Unholy Temple except for the center square, and have a constant stream of 720 mana per turn.
The player's current amount of mana is displayed in blue next to the circular Dungeon Heart health indicator at the top of the GUI.