Intelligence
Intelligence is one of the five base attributes of characters in the game. For all classes, intelligence is the sole attribute that governs the total amount of mana the character will have. See the mana article for how this is calculated.
It is also the primary attribute for mages, meaning it is the attribute that increases attack damage for them. Therefore mages will have many more intelligence points than archers and warriors.
Some special items of equipment will give bonuses to intelligence, including the Master Mage Amulet and Elite Mage Ring. Mages can also increase intelligence with the passive power Dragon's Blood, and archers with Wits.
Intelligence is also believed to increase spell focus and spell resistance. Though to what extent is not yet certain, because very little information on this is provided by the game or NGD.