Discs are pads of fibrous tissue that cushion the spaces between vertebrae (spinal bone). Serving as shock absorbers for the spine, they allow the back to turn and bend normally.  Because the top and bottom of each disc are attached to the vertebrae above and below it, discs cannot actually "slip" out of place.  However, trauma or injury to the spine can cause discs to bulge (herniated) or even rupture (tear).  The resulting pressure on surrounding nerve tissues can be intensely painful.