What is the defining factor?

[Canderis]

What stat/attribute says how often one gets hurt?

[capnnerefir]

Dexterity is the stat that adds to your armor class. Higher dexterity bonus means a higher armor class (unless, of course, you're wearing armor that restricts your dexterity bonus, in which case there is no stat that'll help you). With a higher defense, you're less likely to be hit and, thus, less likely to be harmed.