In more detail
Domain names are read right to left: com is the top-level domain (TLD), example is the name you register and anything to its left, such as www, is a Subdomain. Names are registered through a registrar for a period of years. Each label can be up to 63 characters and the whole name up to 253. Some registries sell names one level down, such as under co.uk; the Public Suffix List records these so that browsers treat example.co.uk, not co.uk, as the registrable domain. Internationalised names with accents or other scripts are stored in a coded form called Punycode.
Why it matters
If a registration lapses, the website and email stop working, and someone else can buy the name and receive mail sent to it. Lookalike names such as examp1e.com are a staple of phishing, so read the domain right to left before trusting a link.
Example
blog.example.co.uk splits into the TLD uk, the public suffix co.uk, the registered domain example.co.uk and the subdomain blog.
Common confusion
A URL is the full address (https://blog.example.co.uk/post?id=7); the domain name is only its host part.