WHOIS is a free lookup service that returns registration details for a domain: when it was first registered, when it expires, who registered it (or the registrar's proxy service, if privacy protection is enabled), and which registrar manages it, all queried against records ICANN requires every registrar to maintain.
It's useful well beyond domain buying and selling. Checking a domain's WHOIS history can help verify how established a site genuinely is before trusting it as a partner or backlink source, confirm ownership during a dispute, or diagnose a technical issue by confirming exactly which registrar and nameservers currently control a domain. Privacy protection services, now standard with most registrars and often included free, mask the actual registrant's personal contact information behind the registrar's own proxy details, which is worth knowing when a WHOIS lookup returns a registrar's name instead of an individual's, that's normal privacy protection working as intended, not necessarily a sign of anything suspicious about the domain itself.
