If you have an existing Ubuntu installation and you wish to install Ubuntu from USB onto another PC, this guide is for you.
Considerations: You can choose either to leave persistent space on the flash drive for user settings or not, depending on how you wish to proceed.
If you don't have a DVD drive on your computer, you do also have other options for installing Ubuntu other than a USB flash drive. One option I've used over the years is an external DVD drive that attaches via USB. It works out of the box on Ubuntu, Windows and OS X. Unlike installing Ubuntu via a USB flash drive, there is nothing to setup, just burn the ISO image to a DVD and install.
Installing Ubuntu is brain-dead easy these days. Whether it's using Ubuntu 64bit for UEFI-enabled PCs or simply running Ubuntu within the confines of a USB flash drive, Ubuntu is accessible just about everywhere.
My advice if you're new to Ubuntu is to try it first via a USB flash drive to see if it's something that works for you. If it's working for you, great, then you can easily install it straight away. If it's not working for you, but you'd still like to keep it around, relying on the USB flash drive is a great way to keep Ubuntu accessible without touching your existing OS installation.