As we know that everywhere there is network of computer/devices connected through internet. when we talk about computer network there is a need to identify a computer uniquely. To identify a computer uniquely in a network a concept was arise known as IP (Internet Protocol address) address, Similar to as we have an unique House address.
The IP address is like '220.127.116.11'. But these number is not easy to remember. So a concept arise that provide mapping of these IP address to a unique name. As we know from our college life to remember a string of character is always easy to remember a long string of number.
So the name which we assign to a IP addrees is known as Domain name.
For example http://www.techie-knowledge.co.in is the mapping of IP address '18.104.22.168'
where techie-knowledge is the domain name
But computer network operate on IP address so there is a service required known as DNS(Domain Name System) server that translate domain name into IP address.