Possible Duplicate:
Place a bootable ISO on a USB drive?
What is the best way to burn an ISO image onto a USB flash drive?
Possible Duplicate:
Place a bootable ISO on a USB drive?
What is the best way to burn an ISO image onto a USB flash drive?
For some variants of Linux, UNetbootin does this without any extra configuration or work.
Depending on the ISO image, I recommend WinToFlash for anything Windows.
For Linux distributions, see Anonymous' answer.
You can use the dd command to "burn" an ISO image to a USB drive from the command line:
dd if=file.iso of=/dev/sdb
Change /dev/sdb to the mount point of the USB drive.
Be careful, as this will destroy the original contents of the USB drive.
Additionally, if the ISO is bootable (such as in installer image) then the USB drive will boot the same way.
You can't really "burn" an ISO image to a USB flash drive.
If you want a bootable USB from an ISO image, there are a couple of things you need to do