When a domain name is registered, specific information about the owner is created and recorded. The info includes contact details such as email addresses, telephone numbers, and postal addresses. All of this is available to the public free of charge, although you can purchase domain privacy protection (often called WHOIS privacy) which will hide this information from the public.
Here are WHOIS database examples:
To search the information through one of those databases, you will need to type a domain name or an IP address into the WHOIS search field. It provides information on any domain name in the world. It is ideal if you are looking to purchase the domain name because it gives you a simple insight into the contact details to get in touch with the site owner.