Mana Capacity = The amount of Mana (new stored value like gold, generated by mana nodes and certain other things, used for spells) a mage can channel in one turn. Say a spell costs 20 mana, and a mage can channel 100... Can cast that spell 5 times if you want. On the other hand, if it costs 200, it will take the mage 2 turns to cast.
Willpower = Determines the amount of summons a mage can control at one time. All summons become permanent, and require both a mana/turn upkeep (run out and the summon vanishes) and a set amount of willpower from the mage. Say a summon is 40 willpower, and your mage has... 100, +20 from a temporary promotion. You could summon a total of 3 (though it may take more or less turns than that, depends on Capacity), but once the temp promotion is up the last summon will turn on you, going barb.