As the PMBOK Guide Fifth edition(Page # 229) says:
"Quality should be planned, designed and built into - but not inspected into the project's management or the project's deliverables. The cost of preventing mistakes is generally much less than the cost of correcting mistakes when they are found by inspection or during usage."
One more simple definition as per PMBOK Guide Fifth Edition(Page # 250) :
Prevention is keeping errors out of the PROCESS , whereas Inspection is keeping errors out of the hands of the CUSTOMER.