Certified Nissan Altima

Certified Nissan Altima?

I am planning to buy a used (05 CERTI FIED NISSAN ALTIMA). Can anyone tell me what EXACTLY a dealership does to certify a used car. Such as, does the dealership replace a part with new parts (brakes, battery, rediator, etc) if the part needs to be replaced? and what assurance do I have in buying a certified car. I've been reading that (IT'S LIKE) buying a new car.

I would like ALL THE INPUT on this before I buy this car.

Thanks,