The WHOIS database provides a certain amount of information for a domain, including but not limited to Registrar, Registrant, contact information, nameservers, etc. The access to the database is complete free and can be done through different sites or terminals, that means that a lot of people can see the information connected to a certain domain name. You can test this yourself if you go to this page http://who.is/ and type in the address you wish to look up. Some domain registrants would rather not have this information displayed publicly, which is why we offer the WHOIS privacy option. For certain domain types you can opt out of having this information displayed during the lookup. 

Please keep in mind that for .UK domains the WHOIS opt-out is free but only available to non-trading individuals. The privacy option is available for other domains as well, however it is restricted to only certain domain extensions (eg. .COM, .CO. .BIZ, etc.) and we charge for this service.