There are four basic ways to create a usb bootable linux.
One is to use a program like unetbootin or usb creator. These applications don't exactly make a real linux system. They don't work on every distro. See www.pendrivelinux.com
Two is to use the live media to usb install if the distro has that. Not all do.
Three is to use a direct to usb with dd command. It will only work on iso images that have been made special called hybrid iso.
Four is the way I tend to use. I use the distro's installer to install directly to the usb just as if it were a real hard drive. (because linux thinks it is a scsi for most distro's)
Also one might use grub or other loader to load directly to the iso file. So that is five.