Whois

WHOIS (pronounced as the phrase who is) is a query and response protocol that is widely used for querying databases that store the registered users or assignees of an Internet resource, such as a domain name, an IP address block, or an autonomous system, but is also used for a wider range of other information. The protocol stores and delivers database content in a human-readable format. The WHOIS protocol is documented in RFC 3912.

Read more about Whois:  History, The WHOIS Protocol, Implementation, Query Example, Referral Whois, Criticism, Accuracy of Information, Law and Policy, Standards Documents