Your hostname is the name of your computer.
Your fully qualified domain name is your hostname plus the domain your company uses often ending in .local.
So if the name of your computer is bob, and your company’s domain is contoso.local, your computer’s fully qualified domain name (FQDN) is bob.contoso.local.:
Hostname: bob
Domain: contoso.com
FQDN: bob.contoso.com.
In the case of a domain like contoso.local I did not use an “external” internet domain name. This name doesn’t have to be the only way that you address the server.
If you make it available by its IP address you can use DNS or that IP address to allow external users to access it.
The dot at the end of the FQDN is used to indicate the empty top-level domain.
Some more information on DNS:
Edit: Thanks for the comment on .local domains RobM