List of internet country codes. also called the country code top-level domain (ccTLD)

ccTLD is an Internet top-level domain generally used or reserved for a country, sovereign state, or dependent territory identified with a country code. All ccTLD identifiers are two letters long, and all two-letter top-level domains are ccTLDs.  

ICANN (Internet Corporation for Assigned Names and Numbers) is the company that regulates the allocation of all ccTLDs. Meanwhile an NIC (Network Information Center, also known as the Domain Name Registry) is the body responsible for the management and registration of all regional domain extensions in a country.

The ISO 3166 standard contains three parts:

ISO 3166-1 – Codes for the representation of names of countries and their subdivisions – Part 1: Country codes[2] defines codes for the names of countries, dependent territories, and special areas of geographical interest. It defines three sets of country codes:
ISO 3166-1 alpha-2 – two-letter country codes which are also used to create the ISO 3166-2 country subdivision codes and the Internet country code top-level domains.
ISO 3166-1 alpha-3 – three-letter country codes which may allow a better visual association between the codes and the country names than the 3166-1 alpha-2 codes.
ISO 3166-1 numeric – three-digit country codes which are identical to those developed and maintained by the United Nations Statistics Division, with the advantage of script (writing system) independence, and hence useful for people or systems using non-Latin scripts.
ISO 3166-2 – Codes for the representation of names of countries and their subdivisions – Part 2: Country subdivision code[3] defines codes for the names of the principal subdivisions (e.g., provinces, states, departments, regions) of all countries coded in ISO 3166-1.