A serial key, which is also known as a product key, activation code, license key or registration key, is a unique string of characters used to authenticate a software copy. A lot of terms can be used to refer to a serial key, all indicating the same concept. The purpose of a serial key is to prevent unauthorized use of the software by requiring the user to enter a valid digital code. In essence, the serial key is a password that unlocks all the features of the software. Whether you call it a license, a registration code, or an authenticator, it all means the same thing - proving you have the software legally and are not a pirate.