(This refers only to the warrior, not the assassin or mage.) 

The warrior's block skill is both defensive and offensive. 

The amount of damage reduction depends on how upgraded the skill is, starting at 30% and increasing thereafter with each upgrade.

To use the skill effectively, hold the skill button down (shield icon) and time your release to the moment your enemy's attack hits your warrior.  The warrior will briefly glow white when the attack hits - release the button at that instant and a powerful counter-attack will be released, knocking your attacker back!