Wikipedia says The Domain Name System (DNS) is a hierarchical decentralized naming system for computers, services, or other resources connected to the Internet or a private network.
The Domain Name System (DNS) is the backbone of the internet which allow users to remember the domain name such as (lcn.com) rather than numeric IP address (such as 220.127.116.11 ).
The right-most label conveys the top-level domain; for example, you can buy a domain name like codementor.io belongs to the top-level domain io.
• Most TLDs with three or more characters are referred to as "generic" TLDs, or "gTLDs".
The primary group of generic top-level domains consists of the com, info, net, and org domains.
Head over to check the complete list of all today existing ccTLDs here