health insurance plan with high deductible and health savings plan?

i am looking into getting a plan with a high deductible for my family ($5000-10000) and a health savings account. does anyone know how this works? I understan the high deductible insrance but not really the health savings part. what happens if you don't use the money? I have a husband and 2 children. right now we have aetna and it costs us $765 a month, what should I do?