Dexterity measures agility, reflexes, and balance. This ability is most important for rogues, but also for characters who typically wear light or medium armor (barbarians and rangers) or none at all (monks, wiards, and sorcerers), or for any character who wants to be a skilled archer.
The dexterity modifier is added to reflex saving throws, armor class, and some attack rolls. The attack rolls for which the dexterity modifier is used are all ranged attacks (unless zen archery allows the wisdom modifier to be used instead) and those melee attacks which benefit from weapon finesse. Furthermore, the dexterity modifier is added to the hide, move silently, open lock, parry, pick pocket, ride, set trap, and tumble skills.
When being added to armor class, the modifier is subject to the maximum dexterity bonus determined by a character's current armor. In addition, the modifier is not added to armor class when a character is flat-footed, unless the character has uncanny dodge or defensive awareness.