Your central nervous system consists of brain and spinal cord.  Protecting the spinal cord are vertebrae (bones) separated by discs made up a gelatin like substance.  The discs function like a shock absorber for the bones and undergo a tremendous amount of stress, which often results in herniation or "bulging" of the disc. This is the most common cause of back pain.

Another common cause of back pain is disc degeneration, caused by a drying out of the disc area.  Over the course of time, or through injury, dried discs will crack and often cause extreme muscle tension and back pain.