USB booting :: n00b with questions

ok, what makes a USB storage device "bootable" (to LINUX) or not?

if there's nothing on the medium... why is it so hard to make it bootable in LINUX?

Is there something hardwired in there to make it NOT compatible with DSL?

If you're starting with a clean slate....so to speak... where does the problem lie?

Sorry if these questions sound stoooopid, but alas, I'm a n00b.