You might be working on a Ubuntu system and might want to create a USB flash drive to install Ubuntu on another computer. Doing this is easy:
First download the ISO file of the Ubuntu version that you want to install.
Plug in the USB flash drive you want to use to install.
Install UNetbootin on your existing Ubuntu system:
$ sudo apt install unetbootin
Open UNetbootin. Ignore the Distribution section at the top. In the Diskimage section at the bottom, choose the path of the Ubuntu ISO file you downloaded earlier. Choose the partition of the USB flash drive as the Drive and press OK.
After UNetbootin is done, unmount the USB drive and plug it into the system where you want to install Ubuntu. Reboot it, you will get a UNetbootin menu, choose Install Ubuntu here and you are on your way to installing it.
Tried with: UNetbootin 2.0.2 and Ubuntu 14.04