A domain name registry is a database of all domain names and their information which is controlled and made available by the registry operator. In the UK for example all .uk domain names are governed by Nominet. Whilst some registries also act as a registrar and allow the direct registration of a domain name, this is typically done by a registrar. Additionally a registry will usually hold all the information of the owners of a domain. 

Read more how registries, registrars, and registrants work together.