1. Attributes
Attributes or so called stats are the basic building blocks for a character's combat ability. The five primary attributes are strength, agility, stamina, intellect, and spirit. These five attributes along with armor appear on the character tab in the menu. These are often referred to as simply stats.
A number of secondary attributes affect specific areas of combat more directly, and are often influenced in some way by a primary attribute. Secondary attributes include critical strike rate, damage absorption, attack power, spell power, and many others.
All characters inherently have some amount of each attribute that increases with level depending mainly on class. For example, a mage will have more base intellect than a rogue, who will have more base agility. Increasing these attributes is mainly done with equipment, as well as temporary effects such as buffs, elixirs, scrolls, auras, and many other means.
Nearly all combat mechanics rely in some way on one or more character attributes. Increasing the appropriate attribute will increase damage done, reduce damage taken, increase healing done, affect a character's mana or health, or other benefits. Understanding how these attributes work is important for effective combat.


2. Strength
Strength has the following effects:
Increases attack power with melee weapons:
Warriors, death knights, paladins, mages, priests and warlocks gain 2 melee attack power per point of strength.
Druids gain 1 melee attack power per point of strength.
Increases parry rating for warriors, paladins, and death knights by 27% of strength. Paladin talent Touched by the Light (Protection, Spec passive) increases spell power by 60% of Strength.
Strength does not affect the chance to land a critical strike. Strength does not improve the chance to block.


3. Agility
Agility has the following effects:
Increases attack power with ranged weapons (not including wands) or melee weapons for certain classes.
Warriors and rogues gain 1 ranged attack power for each point of agility. Hunters gain 2 ranged attack power for each point of agility. Rogues, shamans and druids in Cat Form and Bear Form gain 2 melee attack power per point of agility.
Hunters gain 1 melee attack power per point of agility.
Increases chance to score a critical hit with a weapon.
Increases chance to dodge attacks. This is dependent on both class and level, in fact death knights, paladins and warrios don't.


4. Stamina
Stamina has the following effects:
Stamina provides 1 health per stamina for the first 20 points of stamina, and 14 health per point of stamina thereafter.
Stamina also increases the health of a player's pet for classes with pets, although not from the player's base stamina (the stamina the player has without any armor or talent bonuses). They gain between 3 and 11 health per point of stamina, depending on the pet's type and the master's talents (see the hunter pet, minion and Risen Ghoul articles for details). There are other non-standard adjustments that can be made for stamina:
Death knights -- [Frost Presence] provides an 8% bonus to stamina. The Blood talent tree specialization [Veteran of the Third War] provides 9% stamina. The death knight-only runes Rune of the Stoneskin Gargoyle and Rune of the Nerubian Carapace provide a 2% and 1% bonus to stamina, respectively.
Druids — talent [Heart of the Wild] (Restoration, tier 1) provides a +2%/4%/6% bonus to stamina.[Bear Form] provides +10% stamina.
Hunters — talent [Hunter vs. Wild] (Survival, tier 1) provides a +4%/7%/10% bonus to stamina. Paladins receive 15% stamina from [Touched by the Light] when choosing the Protection talent specialization.
[Blessing of Kings] gives a 5% increase to all base stats (with the exception of spirit), including stamina.
Priests 60 minute buff [Power Word: Fortitude] provides 584 stamina at level 85. Shamans — talent [Toughness] (Enhancement, tier 2) provides a +3%/7%/10% bonus to stamina. Warlocks — talent [Demonic Embrace] (Demonology, tier 1) provides a +4%/7%/10% bonus to stamina.
Warriors receive 15% stamina from [Sentinel] by choosing Protection specialization. Stamina is the key attribute of tanks. It is useful to every class in PvP. Non-tank PvE characters should not actively seek out stamina, but some of it is found on nearly any piece of gear.


5. Intelect
Intellect has the following effects:
Increases spell power by 1 per point at level 80 or above. Intellect is now the main source of spell power for most cases.
Increases mana pool. Each point of intellect gives player characters 15 mana points (except from the first 20 points of Intellect that provide 1 mana for each point instead). Warlock minions gain 1 mana per point of base intellect, imps gain ~4.9 mana for each additional point of intellect, while all other minions gain ~11.5 mana for each additional point of intellect.


6. Spirit
Spirit (often abbreviated SPI) has the following effects:
Increases in-combat mana regeneration for healers. It's the primary attribute for mana regeneration of healers.
Adds 1 spell hit rating per point for shadow priests, elemental shaman and balance druids.
Paladins who pick up the talent Enlightened Judgements gain spell hit equal to 50% spirit (Rank 1) or 100% spirit (Rank 2).
Spirit is of no use for mages and warlocks due to how their classes' mana regeneration works. Spirit is made more valuable by abilities that increase other stats based on Spirit.


7. Secondary Attributes
Secondary attributes are player stats that affect combat directly. The four primary attributes only indirectly affect combat, by contributing to these attributes.
Physical combat:
Weapon damage - Weapon damage is a property of the player's equipped weapon. Melee weapon damage is increased by enchants and sharpening stones. Ranged damage is increased with scopes. Weapon speed - Weapon speed is a property of the player's equipped weapon and determines the time between attacks with that weapon.
Attack power - Increases damage of an equipped weapon. Attack power is increased directly by gear and buffs with the attack power stat, and indirectly by strength and agility (only for hunters, rogues, shamans, and cat-form druids).
Ranged attack power - Increases damage of an equipped bow, crossbow or gun, but not wands. Ranged attack power is increased similarly to melee attack power.
Critical strike - Percentage of attacks that will result in a critical strike. Crit chance is increased directly by critical strike rating, a combat rating, and indirectly by agility.
Hit - Increases chance to hit with melee weapons by a percentage. Hit chance is increased by hit rating, a combat rating.
Haste - Decreases the time between attacks. This effectively increases weapon speed, but has no effect on cooldowns. Haste is increased directly by haste rating, a combat rating.
Expertise - Reduces the enemy's chance to dodge/parry by .25% per point of expertise.
Spell combat:
Spell hit - Increases the chance spells will hit their target. Spell hit is increased by the same hit rating used for melee hit, but the effects differ after that point.
Spell crit chance - Percentage of spells that will result in a spell critical strike. Spell crit chance is increased by critical strike rating and intellect.
Spell penetration - Reduces the target's resistance. Spell penetration is increased directly by gear with the spell penetration rating. It is useful in PvP, not in PvE.
Casting speed - The rate at which spells can be cast. Increased by haste rating. Can lower the global cooldown in cases where spells are cast fast enough to be limited by it.
Defense:
Dodge - Chance to dodge incoming melee attacks. Ranged attacks cannot be dodged. Dodge is increased by dodge rating and agility. Only tanks should use gear with dodge rating.
Parry - Chance to parry melee attacks with a melee weapon. Ranged attacks cannot be parried. Parry is increased by parry rating and strength. Only (non-druid) tanks should use gear with parry rating.
Block - Chance to block melee and ranged attacks with a shield. Warrior and paladin tanks receive additional block chance through their respective mastery and abilities such as Shield Block.
Armor - Decreases physical damage received. Increases your pet's Armor. Armor is always present on equippable items, such as chest pieces and shields. Shields and plate items generally have the most armor, while cloth items have the least. The base armor of an item is determined directly by its type (cloth, leather, mail or plate), item level and slot (chest, head, legs, etc). Some items have extra armor, which is indicated by the total amount of armor appearing in green on the tooltip. Rings, weapons, amulets and trinkets have no base armor, but may have bonus armor.
Resistance - Decreases the damage taken from magical attacks.
Resilience - Decreases the damage taken from enemy players. Resilience is increased through equipment with resilience which is found on PvP gear. Resilience does not have any effect against an npc.