Sometimes in combat you do not want to hit someone so much as to stop them from hitting you.
To use a Parry to block one or more attacks the character uses at least one of their attacks to parry their opponent. If the character wants they can attempt to parry all attacks against them or if they have multiple attacks they can choose to parry one or more attacks while still getting an attack of their own.
There are several different types of parrying maneuvers.
Commonly referred to as the DMG parry. This is an attempt by the character to create a better defense for themselves by giving up all attacks for the round. They can't attack, move, or cast spells. This frees the character to concentrate solely on defense. At this point, all characters but warriors gain an AC bonus equal to half their level. A 6th level wizard would have a +3 bonus to his AC (lowering his AC by 3). A warrior gets a bonus equal to half his level plus one. A 6th level fighter would gain a +4 AC bonus. This benefit is not a perfect all-around defense, and it's not effective against rear or missile attacks. It applies only to those foes attacking the defender from the front. This type of defense has no effect against magical attacks.
A character can declare that they are using one of their attacks to block an opponent's strike. Blocking is a hard parry with a weapon that deflects an opponent's attack. Any weapon except rope-like things such as nets, lassos, or slings can be used to block. When a character tries to block an attack, they make a normal attack roll against AC 4. The opponent makes a normal attack roll against their Armor Class. The strike is blocked if the character succeeds with a lower roll than the enemy succeeded with. If the attacker failed anyway, it's a miss no matter what the blocking character rolled. If the blocking character failed but her attacker succeeded, it's a hit despite the block attempt.
To perform this Parry, you must announce before the start of the round that you're going to Parry. If the character is going to use more than one attack per round for parries you must announce how many of them are going to be Parries. Then, during the round, the first time an attacker strikes at you (even if it's before your turn to strike), you roll your Parry. Roll a normal attack roll against your attacker, and roll vs. his AC (including all bonuses for shield, magical items, etc.). You can use your weapon at its normal chance to attack rolls, or your shield at a +2 to your chance to attack rolls (plus any magical bonus the shield confers). This is in addition to any modifier for Weapon and Shield Style If you hit, his attack is parried and does you no damage. You can Parry thrown weapons, but not missile attacks. Using this parry a character can attempt to avoid special attacks that require a physical touch to complete.
If you're wielding a polearm, you can parry an attack from another character wielding a polearm, even if that character is attacking someone else. To do this, you must be within range either of that attacker or his intended victim.
Missile Weapon Parries
In desperate situations, a character can parry with a missile weapon he is holding (bow, crossbow, or staff sling, but not sling). If he is successful with his parry, though, his weapon is ruined; the enemy's attack has destroyed it. He may continue parrying with it until it is completely destroyed but it may never again be used for its original purpose. Magical missile weapons do not get a save to prevent being destroyed.
Unarmed Combat Parry
A barehanded fighter trying to parry a barehanded attack does so at normal odds.
As with Disarm, it's possible, but dangerous, for a barehanded fighter to parry an attack from a melee weapon. The Parrying character must suffer an AC penalty of 2 and his AC stays disadvantaged until his next attack comes up. Then, when performing the Parry (which usually consists of getting so close to the enemy that the Parrying character can get his own hand under the descending weapon-hand), the Parrying character suffers an additional –2 to attack rolls.