DNS and domains
DNS (Domain Name System)
The directory that turns names like example.lu into addresses, mail servers and other records, run as a tree of zones from the root down.
DNS answers the question "where is example.lu". The answer comes from a chain: the root servers know who runs .lu, the .lu nameservers know which nameservers hold example.lu, and those nameservers hold the actual records. A resolver, usually at your ISP or a public one like 1.1.1.1, walks that chain and caches what it learns.
Records come in types. A and AAAA give IPv4 and IPv6 addresses, MX names mail servers, NS names the zone's nameservers, TXT holds free text used by SPF and DMARC, CNAME makes one name an alias of another, and SOA describes the zone itself. Each record carries a TTL that says how long a resolver may cache it.
The zone is the unit of control: whoever runs the nameservers for example.lu decides every record under it. The registrar only tells the registry which nameservers those are, which is why changing DNS host and changing registrar are two different things.