A domain name is a different name for a website. Domain names must be registered. When domain names are enrolled, they are added to a large list of domain names, and information about your site – including your IP address – is stored on DNS servers.

DNS stands for Domain Name System. DNS servers are responsible for notifying all other computers on the Internet about your domain name and your site address. A domain name in its simplest form is simply a website address on the Internet. Just as your home or apartment has a unique email identification address, each website has its own address.

Domain Registration:

Domains can be registered by domain registration companies such as MailSafi. These companies provide an interface for finding available domain names and various domain name extensions that can be registered at the same time. Domain name registration provides the service for registering .net, .com, .org, .info, .biz, .us, .ws, .nu, .cc domains. 

Image Source: Google

Newer domain extensions such as .info, .biz, and .us offer more options, as many of the popular domains are as yet undefined. Although .net and .com domains are well established and recognized, the most popular domains with this extension are already registered.

Subdomain:

Most people don't know, but they already use subdomains on a daily basis. The well-known "www" on the World Wide Web is an example of a common subdomain. Subdomains can be created on a DNS server and do not have to be registered with a domain registrar. Of course, the original domain must be registered before the subdomain can be created. Subdomains can be requested from your web hosting provider or created by you if you manage your own DNS servers.