A domain name is a name used to locate a site on the web (internet). Let me tell you one thing that, unlike humans, computers communicates by using digits and numbers, called IP addresses. Domains are the user-friendly, readable and memorable names allocated to these numerical IP addresses to make it easy for your users to reach out your site .
Lets take a simple example
If you want to invite me to your home, you have to tell me your address ( Off course, unless how can I find your home ). So tell me how you will do it. Will you tell me that your house is located 120 metres 130 degree west-east from x point. Or will you tell me something like this : Queen Mansion, House No.66, 22B Street, New York City . Clearly, the later one is more preferable option.
Same is the case with websites
It is easy for users to access a site by a domain name – website.com rather than the long numeric IP addresses ( e.g. 145.598.468.489/website/). However computers communicate through these IP addresses, it is not easy for users to memorise them as compare to the domain names. Domains tells the computers that thisdomain.com is hosted on the server xxx. xxx. xxx. xxx
The suffix (.com/.org) after the domain name is called domain name extension.