A domain is an authority within the internet that controls its own resources. Its "domain name" is a way to address this authority as part of the hierarchy in a URL - usually the most memorable part of it, for instance a brand name.
A fully qualified domain name (FQDN) contains all necessary parts to look up this authority by name unambiguously using the DNS system of the internet.
For example, in "developer.mozilla.org":
- "org" is called a top-level domain. They are registered as an internet standard by the IANA. Here, "org" means "organization" which is defined in a top-level domain registry.
- "mozilla" is the domain. If you like to own a domain you have to register it with one of the many registrars who are allowed to do so with a top-level domain registry.
- "developer" is a "sub-domain", something you as the owner of a domain may define yourself. Many owners choose to have a subdomain "www" to point to their World_Wide_Web resource, but that's not required (and has even fallen somewhat out of favor).
- Domain Name on Wikipedia