Blocking is granted by shields and it works much like resistances and armor do in Diablo III. A shield provides another form of damage mitigation, rather than a way to decrease enemy chance to hit. All shields have a % chance to block, and an amount of damage they will absorb upon a successful block. A character's actual blocking % is determined by the shield's number.
Blocking works very differently in Diablo III than in past games in the series. In Diablo III, blocking is basically an item based form of absorb, reducing the damage of attacks by a set amount, one based on the shield being used and potentially modified by item bonuses. This has the effect of making shields much less protective than they were in Diablo II, when any character could eliminate up to 75% of all incoming physical damage with the use of a shield.
A successful block in Diablo III is calculated much as it was in Diablo II, with the character's equipment checked against the attack rating of the enemy.
As resistances provide another form of damage mitigation, one tailored to individual types of magical damage. The biggest change to resistances in Diablo III is that they are no longer percentage-based. They are instead like armor; a number that increases with item and skill bonuses. The higher the number, the more damage they absorb.
This means that characters can always benefit from more resistance, rather than the value capping off at 75%. (Items that add to maximum possible resistance are not found in Diablo III.)