PUK Code (Personal Unlock Key) - definition

There are two (2) types of security feature every carrier company provide in their SIM card which are PIN and PUK protection. These security codes can be found in the back side of the SIM card carrying holder. PIN stands for "Personal Identification Number" and PUK stands for "Personal Unlock Key".
Now, the thing is, an user can lock or protect the SIM card using a 4-digit PIN code. Usually, the carrier company sets a default 4-digit PIN like "1234", but the user has the option to change it further.

Now, the PUK code is usually an 8-digit private key, that can also set by the user (though carrier company provide a unique security code). Because, if the user types the wrong PIN code 3 times then the SIM card locks itself automatically and waits for the PUK code. If the card is again blocked by the wrong PUK code then there is no way to retrieve that SIM card. The only option is left which is to replace the SIM card which will cost the user to loss of all the stored information inside of that SIM card.