FQDN stands for Fully Qualified Domain Name. It is a domain name that specifies its exact location in the tree hierarhcy of the Domain Name System (DNS). It specifies all domain levels, including the top-level domain and the root zone.
Example,
somehost.example.com
hostname -f
A hostname is a label that is assigned to a device connected to a computer network and that is used to identify the device. Example, kafka-dev
$ hostname
AayushTuladhar-Mac.local
/etc/hosts
127.0.0.1 packer-20140710152503.aayushtuladhar.com packer-20140710152503
::1 localhost localhost.localdomain localhost6 localhost6.localdomain6
<ip-addr> kafka-dev.aayushtuladhar.com kafka-dev
/etc/sysconfig/network
[vagrant@kafka-dev ~]$ cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=kafka-dev.aayushtuladhar.com
NOZEROCONF=yes
/etc/init.d/network restart
hostname
hostname -f