Back pain is one of the most ubiquitous illness affecting Americans, costing over $50 Billion and resulting in the disability of thousands of individuals.  In fact, it is the second most common neurological ailment. Visit National Institute of Neurological Disorders and Stroke (NINDS) for more information.

The most frequent cause of back pain is muscular strain.  This is usually a pain that increases with movement and often has an identifiable cause such as lifting a sofa.  Muscular strain is usually to one side or the other rather than in the midline, however it can also occur in the centrally.

Bony injury can also occur.  If there is a history of direct trauma or indirect trauma, only a trained medical professional can determine whether X-rays are needed to identify a more significant injury such as a spine fracture.

Herniated discs are another cause of low back pain that might occurs with relative frequency.  Each of the vertebrae are separated from each other by a spongy disc that can become displaced.  When the disc is displaced, it often places direct pressure on the nerve exiting the spinal cord resulting in numbness and pain in one or the other lower extremity.  This pain with increase with straightening the knee and pulling the affected leg in front of the body.  Surgery is frequently the only solution to this type of chronic and recurring injury, however in some instances, surgery can be avoided.  If there is any focal weakness through the legs or bowel or bladder incontinence or retention, immediate medical care is indicated.

A good source for information is Back.com

Additionally, there are several other causes of back pain that should be discussed.  First, the urinary system can cause back pain such as that pain seen with a kidney stone or kidney infection (pyelonephritis).  In women of child-bearing age, ectopic (tubal) pregnancy or intra-uterine pregnancy can also produce back discomfort.  In the elderly, Abdominal Aortic Aneurysm is another concerning cause of back pain and occurs when the aorta, the largest blood vessel in the body develops a weakening in the wall and a balloon-like out-pouching can occur.  If it begins to tear, it can become painful. Finally, if one develops pancreatitis, back pain with abdominal pain and vomiting can be seen.