Armor works the same way for all classes?

Q u o t e:
Let's say a tier 10 chest piece gives 500 armor. Will a barbarian and a wich doctor benefit equally from it i.e. they get the same amount of damage reduction from those 500 points armor?

Each class gets equal base benefit from armor value on items, however, there are passives and skills which could allow one class to benefit more from armor than another class. For example, barbarians have a passive available to them called Tough As Nails which increases the barbarian's armor by 100%, among other things.