Shield Block
Shields hold three basic stats. Armor, block chance and block amount.
Sacred Shield - Game Guide - Diablo III
Armor is added to your armor mitigation. Every melee and ranged attack has the block chance to be blocked up to the block amount of damage. If the attack holds more damage as the shield can block the additional damage will hit you. The block test happens after the damage reduction through armor and other sources. High armor works very good with blocking. Because of this it is a very good defense option for barbarian.
Total Damage Reduction
For Information or comparing different builds and setups it is often useful to calculate the total damage reduction of your character. Different sources of damage reduction are added multiplicative. Be aware to only use defense mechanics that work against the damage typ/ attack typ you are interested in. This total damage reduction is a median over time as effects as block or dodge are random events.
Basic Formula:
%totaldr=100*(1-(1-dr1)*(1-dr2)*(1-dr3)*(1-dr4).......)
drx= Source of damage reduction
Example:
The barbarian Hoggar has 70% armor mitigation, a shield with 20% block and 10% dodge through dexterity. As a barbarian he also gains 30% damage reduction against every source.
Compared to physical melee damage:
%totaldr=100*(1-(1-0.7)*(1-0.2)*(1-0.1)*(1-0.3))=84.88%
With the assumption that the full damage can be blocked with the shield we see that Hoggar gets only 15% damage from physical melee attacks.
Compared to magic damage dot:
%totaldr=100*(1-(1-0.7)*(1-0.3))=79%
Hoggar is not well prepared to fight magic damage and only achieves 79% damage reduction. He gets ~40% more damage from magic dots than from melee strikes.