What is a domain name?
A domain name is a unique name used to identify and locate computers on the Internet. A domain name provides an easy-to-remember Internet address, which computers translate into numeric IP addresses (Internet Protocol addresses).
An IP address like http://184.108.40.206 is good for computers but difficult for humans to remember. But humans can easily remember a domain name like englishclub.com in http://www.englishclub.com.
NB: The subject of domain names and IP addresses is highly technical. The above explanation is a gross simplification.