The disc consists of a central soft area known as nucleus pulposus and a tough outer ring known as annulus fibrosus. Factors as trauma and degeneration may cause a tear in the annulus fibrosus which will lead to herniation of the nucleus pulposus outwards. This is simply the mechanism of a herniated (slipped) disc.
The herniated disc will start to compress the nearby nerves causing back pain which is accompanied with a sharp lancinating leg pain (known as sciatica and femoralgia).
Mechanism and treatment of herniated disc (lumbar disc prolapse)