WHOIS is a service that allows retrieving information about the owners of Internet addresses (domains) and IP addresses. The English phrase "Who is" means "Who is it" - searchers get information on contact persons of domains and IP addresses.
Depending on the domain, knowledge of the relevant Whois server is required to access stored information. Unfortunately, the queries themselves are becoming more and more complex due to the new endings for different countries and the new generic top-level domains.