How to put Slax to USB drive or disk
Slax for USB is a Slax distribution in TAR format, designed to be unpacked to a writable disk. If you don't know what TAR format is, you may imagine it as a ZIP archive. Extract (unzip) the archive directly to your USB flash key, to an MP3 player or even to a Hard Disk. Most archival programs can extract TAR without any problem. Slax for USB is set to save all changes you make to the boot device (disk), so it will behave like a normally installed operating system.
How to make it bootable
When Slax is unpacked to your device, it will create '/boot/' and '/slax/' directories there. Only one more step is needed to make Slax boot from your USB device or disk: you have to manually navigate to the newly created /boot/ directory and run bootinst.sh script (if you are in Linux) or bootinst.bat script (if you are in Windows).
The method mentioned above will simply install syslinux bootloader to your device's MBR (master boot record). Syslinux works only on FAT filesystem (which is widely used on most USB flash keys, Cameras and MP3 players). If your device or disk is formated using a different filesystem (it is strongly recommended to use a native Linux filesystem like XFS), you will have to use 'liloinst.sh' script instead. Currently, liloinst doesn't provide the same nice boot menu like syslinux, but this will be implemented soon.
The difference between Slax for CD and Slax for USB
Both the variants are exactly the same. The only difference is in the distribution format. ISO is prepared for people who need a bootable CD. TAR is for people who need to extract the contents to disk. But as mentioned, the Slax inside is completely the same. You don't need to download Slax for USB if you have the ISO and you know how to extract it's content. Also, you don't need to download ISO, if you have tar and you know how to make a bootable CD from it.