A domain name is a hierarchical name in DNS. A subdomain sits below another domain: for example, shop.example.com is a subdomain of example.com. Joomla can run on a main website name or a subdomain.
Names, URLs and hosting
Dots separate the parts of a domain name. The name does not include the URL protocol, path or query. Domain registration, DNS service and website hosting are separate services even when supplied by one company.
Creating a subdomain DNS record does not automatically deploy an application. The web server must recognise the name, and its TLS certificate must cover it. Email records also require separate configuration.
Domains in Joomla
When moving Joomla to another domain, check DNS, the server, HTTPS, links, redirects and integrations. A name change can affect cookies, external login services and authentication credentials.
A staging Joomla subdomain does not guarantee isolation from production. Separate files, databases, credentials and external action settings are needed. Domains for language versions belong to the overall URL design.
Conclusion
A domain supplies a name, while a subdomain identifies a place beneath it. A functioning site requires coordinated DNS, hosting and application configuration.