(STR) - Strength increases Physical Attack Power, which increases the damage caused by the Warrior and Monk classes. Strength is the main stat of the Warrior and the Monk.
(DEX) - Dexterity affects Long Range Attack Power, Elemental Defense, and the basic Attack Power of Rogues and long range attack classes such as Archers. Allocating your points to Dexterity can reduce elemental damage received from enemies. The Archer's and Rouge's main stat.
(INT) - Intelligence affects Elemental Attack Power, maximum Mana, and the basic Attack Power of magic-based classes. The Mage and Priest's main stat.
(SPR) - Spirit affects maximum health, Physical Defense, and adds more defense points to your armors. You will naturally gain more health as your level is increased, but investing your points in Spirit adds more health points. Used as a secondary stat for all classes, unless a player opts to only put points into their damage-causing stat.
You gain 4 stat points every time you level up, and 5 per level from level 30 onwards.