The whois data of a domain name provides a lot of useful information.
This includes contact information of the person who owns and administrates the domain, when the domain name expires, the domain name servers, and what IP address or hosting company the domain is currently pointing to.
Many whois tools offer additional information for a fee. For example, they can provide a record of all past whois information for that domain.
This kind of information is very useful if you are trying to get in touch with the owner of a domain you want to purchase or if you want to check the history of a domain name.
Be aware, however, that many domain name owners pay for a feature known as Domain Name Privacy that hides all of their personal information; and despite not being allowed to do so, many people also enter false information when they register a domain. This can make finding the owner of a domain much more difficult.
This page lists a selection of whois lookup tools that will help you see the whois information of a specified domain.