What is WHOIS?
Whois is the internet service that allows users to search a database of domains maintained by NIC.
Whois tool shows whether the domain is registered or not. If it is not registered yet (free) you will be able to register it.
If the domain is already registered, the whois shows the name, address, phone number and email address, etc.. of the person in charge of it, and the sign up and expiration date of the domain, DNS servers, registrer agent, etc..