Mana is a magical power resource available to casters which they draw upon when casting spells. It has some parallels with the warrior's rage, the rogue's energy, and the hunter pet's focus. The amount of mana currently available to the caster is denoted with a special blue mana bar below their health bar. By default, casters start with a full bar of mana which is then depleted as they cast spells.
Characters have an innate replenishment of both health and mana. The rate at which these two stats replenish is based on the Spirit attribute. Some Talents can raise the replenishment rate for these stats independently. Mana regeneration while casting spells is much reduced, but some mana can be replenished instantly during combat using mana potions from alchemists or from various magical items. Out of combat, mana regeneration can be increased significantly by using various drinks. Mana regeneration can also be increased through the use of MP5 gear and buffs.
The maximum amount of mana available to a caster is determined by a combination of the character's class, level and their intellect. In addition, some items (both dropped and player-crafted) grant mana bonuses. Finally, items can be enchanted to give additional increases to mana.
Base mana is a term used to refer how much mana a character has before it is modified by intellect and other item effects. Many spells require a percentage of your base mana to be cast.