In Blizard's Diablo
game series, mana is the spiritual
force flowing within each character's veins
. In Diablo, mana really only had one use: powering spells. In Diablo II
, however, you cannot simply pick up a book
and learn a spell. Each character has a unique set of special skills that he or she can learn, and the largest part of these skills are powered by this mana.
In Diablo, your maximum amount of mana could be increased by spending time (I.E. experience points) developing your "Magic" characteristic. The sorcerer, a class specifically geared toward magic-usage, naturally benefits the most from this development.
In Diablo II, the "Energy" characteristic is what boosts your character's mana, and a class more given to actual magic-usage still benefits more. This is well represented by the fact that the "fighting" classes' attack skills generally have lower mana costs than a magic user's powerful attack spells.
In Diablo II, there are also certain monsters that can drain some or all of your mana with each hit they land on you, making life difficult for a character who relies too much on mana for survival/defense.