Comment by tephra

I'm a programmer at the Swedish Internet foundation which runs (or in industry terms: is the registry for) the .se and .nu (nuie) ccTLDs.

Basically we are a big old database. When someone registers a new domain, via a registrar, they send us that information (most commonly via a protocol called EPP). So we get "this is the domain, registered by this person, it has these hosts, it is registered for this long, with these DS posts, for example. Different registrys have different rules, for example the .se domain has a set of allowed characters that you are allowed to use in your domain name, and rules on how long into the future a domain can be registered for.

This data is then used to perform our most important job as a registry. Creating the zone file for our TLDs so that people could actually reach the domains.

That is from the technical side. There's also a big support side with different aspects. There are abuse issues that need to be taken care of, ICANN has a dispute mechanism that people are involved in.