A product key is used to verify that a copy of a software program is legitimate and has not been installed on more computers than the license agreement allows. It helps prevent the unlicensed use of software. An activation key is specifically used for activating software but does not serve as proof of purchase or ownership which the product key provides. Serial numbers may be unique to each copy of the software but do not serve the specific purpose of verifying the validity of the software license as product keys do. A password protects access to an application or system but does not verify the software's legitimacy.