Domain

A domain name is an identification label that defines a realm of administrative autonomy, authority, or control in the Internet. Domain names are hostnames that identify Internet Protocol (IP) resources such as web sites. Domain names are formed by the rules and procedures of the Domain Name System (DNS).
Domain names are organized into Sub domains and root domains.

example:
Domain

http://www.example.com

sub domain

http://www.support.example.com

The first-level set of domain names are the top-level domains (TLDs), are called generic top-level domains (gTLDs), such as the domains com, net and org, and the country code top-level domains (ccTLDs).

Below these top-level domains in the DNS hierarchy are the second-level and third-level domain names that are typically open for reservation by end-users that wish to connect local area networks to the Internet, create other publicly accessible Internet resources or run web sites.

Domain name registration

Domain names can be registered by domain name registrars.They are assigned by Internet Corporation for Assigned Names and Numbers (ICANN), the organization charged with overseeing the name and number systems of the Internet.

ICANN publishes the complete list of TLD registries and domain name registrars. Registrant information associated with domain names is maintained in an online database accessible with the WHOIS service. For most of the 250 country code top-level domains (ccTLDs), the domain registries maintain the WHOIS (Registrant, name servers, expiration dates, etc.) information.