A domain name registrar is an organization or commercial entity, accredited by the Internet Corporation for Assigned Names and Numbers (ICANN) or by a national country code top-level domain (ccTLD) authority, to manage the reservation of Internet domain names in accordance with the guidelines of the designated domain name registries and offer such services to the public.

In computing, a name server (also spelled nameserver) consists of a program or computer server that implements a name-service protocol. It maps a human-recognizable identifier to a system-internal, often numeric, identification or addressing component. The most prominent types of name servers in operation today are the name servers of the Domain Name System (DNS), one of the two principal name spaces of the Internet. The most important function of these DNS servers is the translation (resolution) of humanly memorable domain names and hostnames into the corresponding numeric Internet Protocol (IP) addresses, the second principal Internet name space, used to identify and locate computer systems and resources on the Internet.

WHOIS (pronounced as the phrase who is) is a query/response protocol that is widely used for querying databases in order to determine the registrar or assignee of an Internet resource (such as a domain name, an IP address block, or an autonomous system number)
WHOIS lookups were traditionally performed with a command line interface application, but now many alternative web-based tools exist. WHOIS services are typically communicated using the Transmission Control Protocol (TCP). Servers listen to requests on the well-known port number 43.
WHOIS has a sister protocol called Referral Whois (RWhois).
Thin and thick lookups
WHOIS information can be stored and looked up according to either a "thick" or a "thin" data model:
Thick
one WHOIS server stores the complete WHOIS information from all the registrars for the particular set of data (so that one WHOIS server can respond with WHOIS information on all .org domains, for example).
Thin
one WHOIS server stores only the name of the WHOIS server of the registrar of a domain, which in turn has the full details on the data being looked up (such as the .com WHOIS servers, which refer the WHOIS query to the registrar where the domain was registered).

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