A JBD is a bootable CD that, in turn, boots a Slackware Linux system that is on a device connected to a personal computer.  That device can be an internal drive, a flash drive (not recommended), or a drive in a USB enclosure.  I have tried these.  It can probably also be many other things that I haven't tried.  A JBD also serves as a handy Live CD for rescue, repair or whatever purpose you find for it.

A JBD changes the relationship of Slackware users to personal computers (PCs).  It allows you to walk up to almost any PC, connect your data device (usually a USB drive), and boot the Slackware OS that is on a root partition of the data device.  This is empowering and, even, life changing.  I can take my USB drive with me to some location (work, my wife's office, my laptop in the kitchen) and boot my familiar Slackware OS.  All my files, software, and customizations are there.  The booted OS appears and behaves very much the same as usual (there may be noticeable differences in performance and the available peripherals.)  My familiar computing environment is no longer tied to the anchor of a single PC.

After the above plug telling how great a JBD is, I need to tell you also of its drawbacks.  Its use is currently limited to Slackware and PCs.  It doesn't work with all PC hardware.  PCs that don't have USB 2.0 or better will experience slow data transfer rates.  Although a single JBD will work to boot multiple root partitions, it only works for root partitions that use a kernel that matches the kernel (and corresponding modules) that have been installed to the JBD.  As a result every time you upgrade to a new kernel you will need to make or get a new JBD to boot your OS.

I invented JBDs, but very little in a JBD is my own work.  A JBD is mostly an assembly of existing methods.  It uses an initrd.gz file made by the Slackware mkinitrd script, and it is made bootable by use of Isolinux or Legacy Grub.  The way that I make a JBD, however, is my own invention.  Except for mkinitrd, all of the scripts that I use to make a JBD are my own.  The scripts are my own, but it needs to be said that I obtained essential guidance from Internet research including, especially, support from the alt.os.linux.slackware Usenet group.