The bones of the spine (vertibrae) have cartillage discs between them to allow flexibility without bone end rubbing together
A slipped disc is when the soft part of the disc bulges through the circle of tissue. This bulge may push on the spinal cord or on the nerve roots. However, it is worth noting that 20 per cent of the population have slipped discs without experiencing any noticeable symptoms.
The term 'slipped disc' does not really describe the process properly - the disc does not actually slip out of place, but bulges out towards the spinal cord.