differences between hostname and fully qualified domain name(FQDN)

Let me use www.youtube.com as an example:

  Aliases :  www.youtube.com
 Hostname :  youtube-ui.l.google.com
     FQDN :  sa-in-f93.1e100.net
Addresses :  ['74.125.200.93', '74.125.200.190', '74.125.200.136', '74.125.200.91']

What is the relation between hostname and FQDN?

According to this website: kb.iu.edu/d/aiu For a mail server "mymail.somecollege.edu", the hostname is "mymail", the domain is "somecollege.edu", combine together is FQDN. But clearly this isn't the case for www.youtube.com. So what is FQDN exactly and what is the relation between all these names? Thanks!