The purpose of the disc is to act as a shock absorber to the vertebrae, and often times the discs may undergo a great deal of stress under pressure.
A simple analogy that will help you understand the disc is to compare it to a jelly donut. If a sudden weight or pressure is placed on a jelly donut, then the inner filling oozes out as a result of the pressure. Although this is a relatively simple analogy, a herniated or bulging disc is quite similar to this.
The intervertebral disc has a number of outer fibrous rings (annulus fibrosus) that surround and provide support to the inner gel-like disc material known as the nucleous pulposus. When there is a tear in these outer fibrous rings of tissue, the inner nuclear material can now ooze out and may protrude or bulge into the channel where the nerve lies, resulting in painful pressure on the nerve root or spinal cord itself.
In a true herniation there has been a complete tear in the outer fibrous walls of the disc and the inner nuclear gel now escapes and puts pressure on the nerve root or spinal cord. In a bulge or protrusion, the tear in the outer wall is not a complete tear, and results in the disc material bulging or protruding onto the nerve root or spinal cord. In either case, the compromise of the disc tissues due to compressive forces may cause significant symptoms.
When this occurs, lower back and leg pain is often the result.