From Diablo Wiki
Reduction differs from resistance in that it reduces damage by a set amount, rather than lowering the damage taken by a percentage. Nothing is yet known about Diablo III's order of operations in damage calculations, so it's not known how the game will process damage.
In Diablo II, reduce and absorb bonuses were calculated 'after resistance lowered the incoming damage, making them very valuable. For instance, a 100 point fire damage attack would drop to 25 points if a character had 75% fire resistance, and then that 25 points of damage was further lowered by reduction or absorption modifiers.