A registry is an organization that manages one or more top-level domains and distributes them via web hosting providers. The registry also defines the registration conditions for the managed domains.
Examples of registration conditions:
.app - may only be used with HTTPS
.de - the registry may request proof of a German postal address within 30 days
.gmbh - only limited liability companies may register this top level domain
Known registries
nic.at - administers the top level domains .at, .or.at, .co.at
DENIC - administers the top level domain .de
Verisign - administers top level domains such as .com, .net ...