Whois is a system for querying registration data connected to a domain name or IP address. It has long been used by registrars, registries, technicians, lawyers and domain buyers.
A Whois response can include the registrar, registration date, expiration date, domain status, nameservers and contact information. For many extensions, personal data is now hidden or restricted because of privacy regulations.
Whois is useful to understand whether a domain is registered, when it expires, which registrar manages it and whether there are technical or administrative statuses that affect transfer or renewal.