We talk about a hernia or slipped disc when a part of the core (nucleus pulposo) from the intervertebral disc passes across a debilitated part of the fibrous ring that contains it. It is result of the degeneration and loss of the elastic properties of the intervertebral disc.
The discs are fibrous structures like cushions that exist between every two vertebrae and muffle the movements of the vertebral column. Diverse diseases or traumatisms, or simply the age can provoke that the above mentioned discs lose their elasticity and could fragment.
When a hernia exists, this can compress nervous structures and provoke symptoms as pain radiated to extremities or loss of sensibility.