The Internet was originally built on the notion that every computer on the Internet has a unique address – just like a telephone number It is called its "IP address" (IP stands for "Internet Protocol").The IP is a unique number consisting of 4 parts separated by dots. Every computer that is on the Internet has a unique number, which correlates to its domain name.

DNS

For ordinary users of the Internet, a more people-friendly naming system called the Domain Name System (DNS) was invented.
The Domain is essentially a signpost that helps users to find their way around the Internet e.g. www.webprospain.com. Almost every website you've ever been to, and every email you've ever composed, has used a domain name in its address. People register domain names in order to 'stake a claim' to a particular name. Once you register a domain name with a registrar, it is entirely yours, as long as you continue to pay the yearly renewal fee.