Bulging discs are not uncommon, and frequently are displayed on MRIs as an abnormality for both young and older adults. Having a bulging disc is not necessarily a serious concern, and it may not even cause back pain. Bulging most likely happens as the body ages and degeneration of the intervertebral disc occurs. A bulging disc is formed when the soft, spongy center of the disc, the nucleus pulposus, pushes out and places pressure on the outer surrounding fibrous ligament, the annulus fibrosis that contains the center. Unlike a herniated disc, the bulging disc still contains the nucleus material. A bulging disc can be problematic when the protrusion against the annulus enters into the space of the spinal canal. If bone spurs form on the facet joints behind the bulging disc, the combination of the bulging and spur growth can narrow the spinal canal. This occurrence is referred to as spinal stenosis. 
Disc disorders are contained or non-contained. A bulging disc is an example of a contained disc disorder. A bulging disc has not broken open; the nucleus pulposus remains contained within the anulus fibrosus. A bulging disc could be compared to a volcano prior to eruption and may be a precursor to herniation. The disc may protrude into the spinal canal without breaking open. The gel-like interior (nucleus pulposus) does not leak out. The disc remains intact except a small bubble pops out attached to the disc. 
A non-contained disc is one that has either partially or completely broken open; a herniated or ruptured disc. To illustrate imagine a tube (anulus fibrosus) of toothpaste (nucleus pulposus) placed under pressure. The pressure causes the toothpaste within the tube to move wherever it can. If any part of the tube is weak toothpaste may leak out. When a disc herniates the contents may spread out to the spinal cord and nerves. The disc material has little space to go --- into the area occupied by the spinal canal and nerve roots. 
Returning to the leaky tube of toothpaste, the disc's gel-like nucleus contains a chemical that irritates the nerves causing them to swell. After the chemical agent has done its job, the remnants of the chemical remain and continue to press on the irritated and swollen nerves. To complicate matters, sometimes fragments from the anulus (tire-like outer disc wall) break away from the parent disc and drift into the spinal canal. These free fragments may travel in the spinal canal. Depending on the type of injury and the condition of the discs, more than one disc may herniate, rupture, or bulge. Sometimes injury causes a combination of disc disorders.