Mana Steal is a frequently encountered trait on items (generally weapons). It allows the user to replenish their Mana with damage dealt against the enemy.
The steal comes from how much damage you do to the monster. So 5% Mana Stolen will return 5% of the damage you actually deal, rather than being based on the damage your weapon deals. Life and Mana steal only work on Melee, and physical ranged attacks.
There are only 2 forms of Mana Steal in Diablo I/Hellfire. They are 3% Mana Steal, and 5% Mana Steal. Neither will stack for a Bard dual wielding weapons with those affixes. Only the most powerful of the combination applies.
Diablo II/Lord of DestructionEdit
Mana Steal, much like its sibling, Life Steal, is buffed in Diablo II/Lord of Destruction. Melee and Ranged physical attacking heroes seek out this affix so that they can restore the Mana they expend for their Skills, allowing them to constantly use them against the minions of Hell. Heroes can find this on Weapons, Jewelry, Circlets, and Gloves. Unlike Life Steal, however, it is not so stacked highly. The reason for this is because the Mana expenditures tend to be little, versus the insane physical damage one dishes out per hit. Changing difficulties will reduce the effectiveness of Mana Steal. Comparing the changes to Normal, Nightmare cuts Mana Steal in half, and Hell Difficulty will reduce effectiveness to 1/3rd listed.