Degenerative Disc Disease (DDD) is not a "disease" per se, but rather a term used to describe a process or condition that develops gradually and worsens over time. The use of this term indicates that cartilage-like discs between the spinal vertebral joints are the primary cause of the symptoms, and that the degenerative changes are rather advanced.
To some degree, inter-vertebral discs lose their flexibility, elasticity and shock absorbing characteristics as we age, as do other tissues in the body. Abnormal or excessive mechanical stresses or injuries of the past coupled with hereditary, developmental and metabolic influences can rapidly accelerate this process.
As the involved disc dries out and loses height, it causes the vertebra to become closer together, narrowing the channels through which the nerve roots pass. A dry, thin disc is unable to do its job of absorbing shock, further stressing the joint as well as the supporting muscles and ligaments. The worse it gets, the poorer its shock absorbing capability, which exponentially accelerates the process.
Contrary to what one might suppose, stress to these tissues does not result in a wearing away, but instead leads to a gradual buildup of calcium deposits otherwise known as osteoarthritis. This is similar to how stress to the skin on the palm of your hand results in a buildup of skin cells otherwise known as a callous. This progression of disc, thinning and osteoarthritis is called degenerative disc disease. The resultant narrowing of space available for the spinal cord and spinal nerve roots is called spinal stenosis.