Dexterity measures hand-eye coordination, agility, reflexes, and balance. This ability is the most important one for rogues, but it’s also high on the list for characters who typically wear light or medium armor (rangers and barbarians) or no armor at all (monks, wizards, and sorcerers), and for anyone who wants to be a skilled archer.
You apply your character’s Dexterity modifier to:
-Ranged attack rolls, including those for attacks made with bows, crossbows, throwing axes, and other ranged weapons.
-Armor Class (AC), provided that the character can react to the attack.
-Reflex saving throws, for avoiding fireballs and other attacks that you can escape by moving quickly.
-Balance, Escape Artist, Hide, Move Silently, Open Lock, Ride, Sleight of Hand, Tumble, and Use Rope checks. These are the skills that have Dexterity as their key ability.
- Touch AC
- Ranged touch attacks for spells
Archers and rogues will want a Dexterity score of around a 15 minimum, and this is usually recomended to be the highest stat.
Other characters usually put this stat at 14 for the highest. It is commonly the lowest stat for heavily armored warriors. Arcane and divine casters will want a decent Dexterity score to help with their low AC, and their ranged touch attacks.
For other uses of Dexterity, Check out this thread on the WoTC boards X Stat to Y Bonus