The Arreat Summit has a bunch of very useful information on this, as well as other topics. Check this out:
The Arreat Summit - Basics: Character Information Quote:
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Originally Posted by Arreat Summit on Defense Defense Rating - This reflects your ability to dodge attacks in combat. The higher the value, the less often your character will be hit. Highlight your Defense Rating with your cursor to display the percentage change a creature would need to hit you successfully for the last monster you have fought.
If you remove all equipment that affects your Defense Rating, your Defense Rating = Dex / 4 (round down).
Melee attacks will never have less than 5% or more than 95% chance to hit.
Will a higher Defense Rating reduce the amount of damage I receive when I'm hit?
No, a higher Defense Rating reduces the chance that monster will hit you.
I put on armor that has a Defense Rating of 100, but my character's overall Defense Rating increases by 200. Why?
Your Dexterity bonus might account for this, as might passive skills (such as Barbarian Iron skin) or any magical bonuses that boost Defense Rating. |
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Originally Posted by Arreat Summit on Block Blocking
Blocking determines your chance to defend against physical melee and ranged attacks. If you block an attack you will receive no damage. If you run your mouse over your Defense on the Character Screen you will see a percentage to block if you are carrying a Shield or Necromancer Shrunken Heads. If you do not have a shield or Shrunken Heads you will not see this listed.
When a player blocks, that is, after a hit has already occurred, probability is computed that a player will block as follows:
Total Blocking = (Blocking * (Dexterity - 15)) / (Character Level * 2)
Blocking = A total of the Blocking on all of your items.
To raise your % Chance to Block spend more points in Dexterity in addition to getting higher % Chance to Block via items. The block value itself is a combination of a value inherent to that particular player class, and any other block bonuses from items. This value is capped at 75%. If the roll of the dice denies a true block, or if the player can't block at all, then and only then, are skills such as Amazon Avoid and Assassin Weapon Block checked for blocking.
Note that if a player is running, the Total Blocking percentage is reduced to 1/3rd of its original value, with a cap of 25%. |
So in general, they both reduce the chance for opponents to hit you, the major difference is that with block all opponents will have the same chance of being blocked regardless of their level. With defense rating, the higher the opponent's level the better their chance of hitting you anyway.