When you register a domain name all the header's personal data are registered in a "global database". This database collects all the informations about the company or the individual such as name, surname, address, telephone number, e-mail. All the informations about a domain header can be recovered making a Whois.
If you want to hide your personal data or part of them from Whois you have to activate the ID protection option in your domain. In case of domains with generic extensions (such as .net, .org, .com, .info, etc.) all your data will be obscured on Whois. In case of a domain .it and domain .eu the ID protection option will be activated activated by default. In case of domains .it only individuals are allowed to use the obscuring of data. Otherwise companies cannot use the same option anymore.