The discs between each vertebra are made of a strong outer covering that encloses a gel-like center. This design allows the discs to serve as shock-absorbing cushions that support and stabilize your spine.
Over the years, discs gradually dehydrate and weaken due to postural imbalances, spinal instability, and wear-and-tear degeneration. A disc can also become damaged due to an injury, such as fall. Once a weakness in the outer covering develops, pressure from your spine pushes the center of the disc through that weak area.
In some cases, the disc only bulges out. However, a true herniated disc occurs when the cover ruptures and the inner material leaks out, and creates a chaffing effect on the surrounding nerve roots.