The discs in your spine are cushions between your vertebrae that keep the bones of the spine from touching and also give your spine its flexibility and range of motion. Discs are often compared to jelly donuts because of their tougher exterior and squishy interior.
A herniated disc occurs when a bit of the softer center of the disc pushes through the tougher exterior. The result is usually quite painful since the bulging of the disc often results in impingement of the spinal nerves that run through the entire length of the spinal cord.