The Internet Explained

Domain Name

A domain name is used to give a server a name that is easier to remember than an IP address. A domain (the 'name' part is often omitted) can be bought from a domain name registrar. You can configure a DNS record for a domain. This means the DNS server of the registrar lists a new entry for your domain and your servers IP address. Common DNS records are: A (address), CNAME or ALIAS.

Here is an example of an URL and the different pieces explained:

https://www.roadmappy.com/maps/overview
https:// the protocol HTTP is used (other protocols are file:/// or ftp://).
www. subdomain (no every URL has a subdomain)
roadmappy Second-level domain
.com Top level domain
/maps/overview pathname
www.roadmappy.com hostname

Sometimes you see also the usage of a subdomain (for example: the mobile variant of a website, or a localized version or an app)

https://de.wikipedia.org/ (de. localized German version of Wikipedia)
https://old.reddit.com/ (old. old reddit user interface is still available via the subdomain)
https://books.google.de/ (book.)
there is no limit to possible subdomain names...