Strength affects your Hero's melee and thrown weapon damage. The higher the Strength, the more damage your Hero will cause, and also the more weight your Hero can carry. For fighters, Strength also affects the amount of damage that their Power Moves inflict.
Dexterity affects your Hero's Armor Class (AC) and is a measure of how much damage your Hero will sustain from a hit.
Constitution affects the amount of bonus Hit Points a Hero gains during level up. The higher the Constitution, the higher the HP bonus.
Intelligence affects Mystical Will point gain. The higher the Intelligence, the more Mystical Will your Hero will earn when he or she levels up. Intelligence also affects the strength of the wizard's spells.
Wisdom affects your Hero's Mystical Will recharge rate. The higher the Wisdom, the faster the recharge rate. For clerics, Wisdom also affects spell strength.
Charisma affects your Hero's Level Up Point gain on level up. The higher the Charisma, the more Level Up Points your Hero will receive when he or she gains a level. Also, Charisma affects the price and value of items.