Your spine is made up of little bones called vertebrae. In between these bones there are "discs" that cushion the movement that occurs between each of the bones. These discs have water in them to allow for this cushioning. In the picture on the right the discs are blue. The yellow represents the nerves coming and going to and from your spinal cord.

A herniated disc is when some of the fluid in one of these discs slips out from in-between the vertebrae. The herniated disc can press backwards onto the spinal cord or any of the nerves that come off of the spinal cord and go to your skin and muscles.