Your hostname is le name of votre computer.
Your fully qualified domain name is votre hostname plus le domain votre company uses often ending in .local.
So si le name of votre computer is bob, et votre company's domain is contoso.local, votre computer's fully qualified domain name (FQDN) is bob.contoso.local.:
Hostname: bob
Domain: contoso.com
FQDN: bob.contoso.com.
In le case of a domain like contoso.local I did pas use an "external" internet domain name. This name ne have to be le seulement way that you address le server.
If you make it disponible by its IP address you can use DNS ou that IP address to allow external users to access it.
The dot at le end of le FQDN is used to indicate le empty top-level domain.
Some more information on DNS:
Edit: Thanks for le comment on .local domains RobM