A domain name that you can register with a registrar is usually one level from a top-level domain. For example, drtak.org is one level down from the top-level domain org. The registration process makes sure that a domain name is issued only to one party.
Most domain names can be registered easily as long as they are not reserved. However, certain top-level domains, such as edu and gov, are reserved.
Once a domain name is registered, the registrar reserves the domain name. The registration process also asks about the designated primary domain name server of the domain. This can be provided as a domain name or an IP address.