There are a number of ways that you could install Ubuntu on a USB drive and it all depends on what you want. Do you just want a live environment on the USB drive? or do you want to be able to save documents and make changes to the settings and have them persist across sessions? Or do you just want to treat the USB drive as if it was a hard-drive and do a traditional install on there instead?
Also, do you plan to do this from Windows or Linux?
Whatever you plan to do, here are several ways that I have used in the past to get Linux live .iso's onto USB:
If you boot to the desktop via a Ubuntu live CD/DVD, there should be a program called something like USB creator (can't remember the exact name, but it should be on the LiveCD, do a search for it in the HUD). This will allow you to create a live USB install from the CD/DVD you booted from.
If you plan to do this from Windows, then unetbootin is pretty much your only option!
With USB Creator and unetbootin, you can also set up persistent storage (this allows you to set aside some space on the USB drive for a virtual file-system which allows you to save personal documents and settings so they persist between sessions) or you can go without the persistent storage and just use the USB media as if it was a LiveCD.
Another option would be to install Ubuntu on the USB drive as if it was an ordinary hard-drive. In which case, you boot via liveCD/DVD and run the installer and install Ubuntu onto your USB drive instead of your main HD - but you need to do this VERY carefully or you might accidentally overwrite your main HD!
Another way of creating a live USB (without any persistence) would be to use the dd command from the terminal in any linux desktop session: dd if=/path/to/ubuntu.iso of=/dev/sdb -bs=1M
Where /path/to/ubuntu.iso is the path to the iso file you want to copy to USB and /dev/sdb is the USB drive you want to copy to.
dd basically clones the iso onto the USB drive, so it is identical to the liveCD/DVD iso.
NOTE: in the dd command listed above, /dev/sdb was just an example. You need to substitute that for whatever device node is assigned to your USB drive. dd is a dangerous command, if you output to the wrong drive, you could seriously mess up your system! dd will just write whatever it is told to wherever it is told! You have been warned! :) Some people have a nick-name for dd: "Data Destroyer"! Heh heh!
The best way to determine the correct device node would be to check the output of the df -h command. Look at the drives listed and you should be able to determine which drive is which from their sizes and the amount of space free on them.
I don't use Windows, I'm ubuntu user since version 9.01.
I'll upgrade my hardware (board, cpu & memory) and planing to install Ubuntu 14.04 64 bits version.
Actually I have Ubuntu 12.04 32 bits and did download the Trusty Tahr iso, did backup my data and I'll do a clean isntallation, that's the reason for my request.
But reading the Ubuntu site I'd found this ---> http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-ubuntu
and will try it and tell here my experience
Will take note of your coments.
Thanks a lot!
As CimmerianX said Universal USB Installer is best. You can also use YUMI (which you can find on the above link). The advantage is, you can add multiple ISOs to your usb Drive. Also you can use GRUB4DOS, which is available for win and linux Distros.