When you register for a domain all your personal information including your address, telephone nos. and the people or organizations associated with the domain are listed publicly . So it is accessible to anyone who per chance tries to register for a domain with the name similar to one already registered. All this info is listed in a directory called the WHOIS directory which is a public listing of domain names.

What is WHOIS?
With the presence of numerous web-based directories online, it is very easy to procure information about domain owners and IP addresses. All this info (mail address and phone nos.) is stored in the WHOIS directory.

While buying a domain you can check the Private Registration option, wherein all your personal data is made private. So for domain owners who would rather have their personal information hidden from public view, this option serves the purpose. When you check the option, your personal information will not be stored in the WHOIS directory.