Whois is a tool (both application and web application level) which is used to gather information about target domain like name server,domain records,admin contacts and other relative information .
Whois is one the major information provider and this information is used in writing penetration testing reports.It is a great database which contain records of almost every domain name.
www.whois.sc is one of the popular website to check whois information
How to use whois.sc
1. Navigate to www.whois.sc