Blocking is one of the game mechanics that prevents an attack from hitting. A blocked hit deals no damage, yields no adrenaline to the attacker nor the blocker, and is visually represented by the text "block" floating above the target's head seen by the attacker along with a distinct sound of a muffled hit. There are a variety of skills that provide blocking, usually either cast on an ally or yourself. These are generally used to defend a character under heavy fire from physical sources (such as Warriors and Rangers) since they do not prevent the damage from non-attack skills.
However, it should be noted that many skills can punish a target for blocking, providing extra damage, knockdowns, or other penalties if the attack is blocked.
Other skills bypass blocking or disallow the target to block.
There is no cap on stackable blocking skills, but the effects are multiplicative. This means that two 50% chance to block skills do not add up to a 100% chance to block. Rather the first block skill applies, and any attack that bypasses that has to pass the second block skill for a total effective chance to block of 75%.