A disc is a cartilage like structure that lies between the vertebrae (bones) in the spine. The discs act as cushions for the vertebrae and space creators for the nerves that exit through the openings in the spine. They contain a gel like structure in their middle to help absorb and distribute shock. Without these discs the bones (vertebrae) of the spine would sit on top of each other and crush the nerves causing severe pain and dysfunction.
When too much localized pressure is applied to the spine the gel can be pushed out of the middle of the disc and into these openings for the nerves. The nerve gets distressed causing pain.
Localized pressure can be caused by trauma to the spine (eg. a car accident) or long term compression in the spine. More often than not a disc becomes herniated (or torn) because the muscles of the spine have been chronically contracting and forcing the vertebrae together decreasing the space between them. This compression over time wears out the outside layers of the disc creating an environment where they can easily be herniated.
They key to good disc health is to create an environment in the spine where the vertebrae have an abundant amount of space and the muscles are not chronically contracting.