Pictured here is a herniated disc in the lumbar region.  The disc sits between two vertebral bodies, and acts as a cushion or shock absorber.  When the firm covering of the disc (annulus fibrosis) is fractured or injured, the soft interior (nucleus pulposus) can squeeze out, like toothpaste, and as seen here, can push upon the nerve root.  While the picture here shows a herniated disc within the lumbar spine, it can happen in the cervical spine as well, and less frequently in the thoracic spine.