Dexterity, DEX in your 'score', represents your character's agility and physical skill.
Dexterity is very important for quaffing potions and eating pills during combat. High dexterity will allow a character to quaff or eat without dropping many potions or pills during the heat of battle. Poor dexterity will causes a character to accidentally drop vials or trample pills before they can be consumed.
Among other things, dexterity affects:
Dexterity is the prime attribute of the Thief, Vampire, Fathomer, and Bladesinger.
*Information as of July 27, 2005