http://www.wowwiki.com/Stats
Strength (often abbreviated STR) has the following effects:
Increases attack power with melee weapons:
Warriors, shamans, druids and paladins gain 2 melee attack power per point of strength.
Rogues, hunters, mages, priests gain 1 melee attack power per point of strength.
Slightly increases the amount of damage blocked with a shield for shield-using classes (warrior, paladin and shaman)
Agility (often abbreviated AGI) has the following effects:
Increases attack power with ranged weapons (not including wands) or melee weapons for certain classes.
Warriors, hunters and rogues gain 1 ranged attack power for each point of agility.[1]
Hunters,[2] rogues and druids in Cat Form gain 1 melee attack power per point of agility.
Increases armor by 2 per point.
Increases chance to score a critical hit with a weapon.
Increases chance to dodge attacks. This is dependent on both class and level.
Stamina (often abbreviated STA) has the following effects:
Stamina provides 10 health for each point for all Classes/Mobs/NPCs (except from the first 20 points of Stamina that provide 1 health for each point instead). It also increases health points of a player’s pet for classes with pets. There are other non-standard adjustments that can be made for stamina:
Intellect (often abbreviated INT) has the following effects:
Increases mana points. Each point of intellect gives 15 mana points (except from the first 20 points of Intellect that provide 1 mana for each point instead).
Increases chance to score a spell critical strike. This increase varies by class and level; in general, the relationship is linear as intellect increases, with a small amount of base crit chance. The rate of increase is generally based on the amount of intellect a class is expected to have at a certain level, given by the following table.[3][4]
As of 2.4, Intellect will increase the amount of MP5 generated by Spirit.
Spirit (often abbreviated SPI) has the following effects:
Increases health and mana regeneration. Spirit affects all characters’ mana and hit point regeneration rates in and out of combat.
Health regeneration from spirit is not active while in combat, except for trolls who gain 10% of their normal health regen in combat from the racial ability “regeneration.”
Mana regeneration is active in combat except for the five seconds immediately after casting a spell (the five second rule). Some class talents and spells allow some mana regeneration during this period.