A domain name usually consists of two or more parts (technically labels), separated by dots. For example wikipedia.org.

* The rightmost label conveys the top-level domain (for example, the address en.wikipedia.org has the top-level domain org).
* Each label to the left specifies a subdivision or subdomain of the domain above it. Note that "subdomain" expresses relative dependence, not absolute dependence: for example, wikipedia.org comprises a subdomain of the org domain, and en.wikipedia.org is a subdomain of the domain wikipedia.org. In theory, this subdivision can go down to 127 levels deep, and each label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 255 characters. But in practice some domain registries have shorter limits than that.
* A domain name that has one or more associated IP addresses is called a hostname. For example, the en.wikipedia.org and wikipedia.org domains are both hostnames, but the org domain is not.

The DNS consists of a hierarchical set of DNS servers. Each domain or subdomain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains "beneath" it.

Thanks for this, definitely helpful.
I never really thought about the details and technical specifics before.