You're quite right when you point out "back pain" is a symptom of a problem--not the real problem. Many patients with back pain don't have a known cause of the problem. There's no infection and no fracture.
Mechanical back pain may help further describe or isolate where the problem is located. This diagnosis suggests a dysfunction in the way the spine moves. Perhaps the joint is arthritic or the ligament is stretched out. There may be degenerative changes in the discs or spine causing pressure on the spinal nerves as they exit the spinal column.
Researchers have actually isolated a direct link to mechanical low back pain in many patients. It turns out psychosocial factors of stress, poor work conditions, and economic troubles may be more likely to start back pain than any real physical condition.