"This is useful if you want to install Ubuntu on a
computer that has no CD/DVD drive. When you create the USB startup
disk, you can also specify that you want your USB system to be
persistent between boots (i.e., it does not lose your settings,
documents, etc.) - that way you get a fully usable OS that you can
carry around in your pocket...
"The Make USB Startup Disk tool starts. It should make the right
choices for the source CD and the target USB drive automatically
(if you want to use the Ubuntu 8.10 .iso file, click on Other...
and select it from your file system). If you just want to create a
USB startup disk that you need for installing Ubuntu 8.10 on
systems without a CD/DVD drive, select Discarded on shutdown,
unless you save them elsewhere - the startup disk will then behave
like the Live-CD, i.e., it will lose all settings and documents
between reboots. If you want to have a "full" OS that saves your
changes between reboots and that you can carry around in your
pocket, select Stored in reserved extra space and specify how much
space on your USB flash drive should be reserved for your documents
and settings. Click on Make Startup Disk afterwards:"