This condition has many names, slipped disc, ruptured disc, prolapse.  The vertebrae of the spinal cord are separated by disks made of cartilage. The disks have a strong outer layer and a softer inner part that acts as a shock absorber to cushion the vertebrae during movement.  A herniated disk is a result of the inner softer part of the disk bulging out through a weakness in the outer part of the disk. The bulging disk may press on nearby structures such as a nerve coming from the spinal cord. Any disk in the spine can become herniated however, most herniated disks occur in the lower part of the spine. 

A herniated disk between vertebrae is the most common cause of nerve root damage.  The collapsed vertebrae may result when bones are weakened by cancer, osteoarthritis or severe injury.  Activities increasing the risk of developing a herniated disk include lifting or excessive periods of sitting.  How badly the nerve root is compressed determines how severe the pain or other symptoms will be.