In the registering process you must give technical information to locate the entity pointed to by a domain name on the Internet. You need to open an account with a registration service provider. The service provider assigned one or more nameservers that are responsible for translating your domain name to a numerical IP address. Without a nameserver, no one will be able to locate your domain on the Internet.
Reserving a domain name means you are setting aside a name you plan to use in the future and to make it impossible for someone else to use it. It is also called "domain parking".
