New versions of ubuntu comes with a USB creator program which can creat bootable usbs from a downloaded ISO image.
The recommended way to install Ubuntu from a USB flash drive is to use the usb-creator program, point it at a ISO image of a Ubuntu installation CD that you have downloaded, and let it create a bootable USB that you can use instead of the CD.
The process is as simple as copying a file to your usb. Follow the steps given.
1. Acquire the correct Ubuntu installation files ('the ISO') and tools
2. Put Ubuntu onto your USB flash drive
3. Configure your computer to boot from USB flash drive and boot from it
4. Try Ubuntu or install to your hard drive.
Booting from a USB flash drive will behave just as if you had booted from the install CD. It will show the language selection and then the install menu, from which you can install Ubuntu onto the computer's hard drive or launch the Live environment.
It is adviced to have wired internet access while booting from USB, as it is not certain that Ubuntu can recognise a WiFi-wireless card on the first attempt.