A domain name registry is a database of all domain names and the associated registrant information in the top level domains of the Domain Name System (DNS) of the Internet that enables third party entities to request administrative control of a domain name. Most registries operate on the top-level and second-level of the DNS.
A registry operator, sometimes called a network information center (NIC), maintains all administrative data of the domain and generates a zone file which contains the addresses of the nameservers for each domain. Each registry is an organization that manages the registration of domain names within the domains for which it is responsible, controls the policies of domain name allocation, and technically operates its domain. It may also fulfill the function of a domain name registrar, or may delegate that function to other entities.
Domain names are managed under a hierarchy headed by the Internet Assigned Numbers Authority (IANA), which manages the top of the DNS tree by administrating the data in the root nameservers. IANA also operates the int registry for intergovernmental organizations, the arpa zone for protocol administration purposes. IANA delegates all other domain name authority to other domain name registries and a full list is available on their web site. Country code top-level domains (ccTLD) are delegated by IANA to national registries.
Domain name registrar
A domain name registrar is a company that manages the reservation of Internet domain names. A domain name registrar must be accredited by a generic top-level domain (gTLD) registry or a country code top-level domain (ccTLD) registry. A registrar operates in accordance with the guidelines of the designated domain name registries.
Domain registration information is maintained by the domain name registries, which contract with domain registrars to provide registration services to the public. An end user selects a registrar to provide the registration service, and that registrar becomes the designated registrar for the domain chosen by the user.
Information about the Root Zone Database, as Registry and it's Policies, you can find on IANA web-site (https://www.iana.org/domains/root/db).