Every time you make an attack in WoW you will generate a combat event based off the hit table. This article will discuss the different potential combat events and their effects in detail.
A hit is simply whenever your attack or spell connects with your opponent. There are typicially no special damage modifiers for a normal hit - you just attack, and the attack lands. When looking at the attack table, Hit is always the least prioritized event and thus the first eligible to be pushed off.
A critical hit is when your attack lands and deals additional damage, above and beyond what you would see out of a hit. The base multiplier for melee attacks is that critical hits will deal double damage, and spells will deal 150% of normal damage, but many classes have talents that will give bonus effects on landing a critical hit or a higher damage multiplier. For a more detailed discussion of a way to determine your precise hit chance, see here.
A miss is when an attack fails to land completely. Your chance to miss is mitigated by your character's hit rating, and is worsened by your target's defense rating as well as certain talents and penalties if you are dual wielding. By default, your chance to miss a target of equal level is 5%, whereas your chance to miss a boss mob is 9%.
A dodge is when you attack but your opponent evades. Your chance to be dodged is mitigated by your character's weapon expertise, and is worsened by your target's dodge rating and defense rating as well as certain talents. Additionally, players will lose all chance to dodge if attacked from behind but mobs will not. By default, the chance for a mob of equal level to dodge your attack is 5% and the chance for a boss mob to dodge your attack is 6.25%; dodge rating varies wildly between PCs.
A parry is when you attack but your opponent uses his weapon to deflect the blow as well as speeding up their next attack. Your chance to be parried is mitigated by your character's weapon expertise, and is worsened by your target's parry rating and defense rating as well as certain talents. Both players and mobs lose the chance to parry attacks from behind. By default, the chance for a mob of equal level to parry your attack is 5% and the chance for a boss mob to parry your attack is considerably higher; parry rating varies wildly between PCs, and certain classes have no chance to parry attacks.
A block is when you attack but your opponent uses his shield to mitigate a portion of the incoming damage determined by his shield block value. Your chance to be blocked is worsened by your target's block rating and defense rating as well as certain talents and abilities which can raise your chance to block. Both players and mobs lose the chance to block attacks from behind. By default, the chance for a mob of equal level to block your attack is 5% and the chance for a boss mob to block your attack is considerably higher; block rating varies wildly between PCs, and is impossible if your class cannot (or is not currently) wearing a shield.
A resist is when either all or a portion of your spell is prevented on the target. Some spells can only be fully resisted or not at all, whereas most spells can be partially resisted to reduce the damage in segments of 25%. Your chance to be resisted is reduced by your spell hit rating as well as spell penetration to help bypass your opponent's resistances, and worsened by your opponent's resistances and certain talents. By default, the chance for a mob or player of equal level to resist your attack is 5%, but can only be partially mitigated by spell hit; there will always be a 1% chance for any spell to be fully resisted.
A crushing blow is when a higher-level NPC attacks a player and deals 150% of his normal damage. The default chance for a boss mob to land a Crushing Blow is 15%, assuming your defense skill is fully leveled; an underleveled defense skill will lead to much higher chances of crushing blows, but increasing your defense skill above the cap for your level via defense rating will not further reduce crushing blows. Clever use of the hit table can be used by some classes to prevent crushing blows by stacking block chance.
A glancing blow is when a player attacks a NPC and deals a reduced amount of damage, by default 70% of your normal hit damage. The default chance to score a glancing blow on a boss mob is 25%, assuming your weapon skill is fully leveled; an underleveled weapon skill will lead to a much higher chance of glancing blows as well as reducing the amount of damage each hits with. Historically this could be mitigated by stacking weapon skill, but all weapon skill rating has since been replaced by expertise rating.