Registrant is the owner of the domain name and registar is the one who holds the registrants records. In certain cases you can always contact the registar to get further details of domain owners like who is the registrant or registered domain owner.
Whois is a query tool that uses response protocol that queries the databases of InterNIC and fetches the registrant of the domain name or Ip address along with the registrants contact details. Moreover it also display the details of the name server, record creation date and expiration date of the queried domain name.
Check who is domain or ip address below