Armor: An isolated, static ability that means, "This unit can only be hit on a roll of 5 or more." Armor works against dice of damage and attack dice. It redefines what die roll is required to hit a unit. It doesn’t modify the die roll like Accuracy. A unit with Accuracy 1 attacking a unit with Armor hits with 4's, 5's, and 6's.
Shields
Shields X: A cumulative, static ability that means, "The attacking unit gets –X power against this unit." Shields only matters when attack dice are rolled. If you give a unit Shields after your opponent has rolled for damage, it doesn't affect the roll.