Chronic pain is different than acute pain (a normal sensation triggered in the nervous system to alert you to possible injury and the need to take care of yourself), as it persists. Pain signals keep firing in the nervous system for weeks, months, even years.

There may have been an initial mishap—sprained back, serious infection, or there may be an ongoing cause of pain—arthritis, cancer, ear infection, but some people suffer chronic pain in the absence of any past injury or evidence of body damage. Many chronic pain conditions affect older adults. Common chronic pain complaints include headache, low back pain, cancer pain, arthritis pain, neurogenic pain (pain resulting from damage to the peripheral nerves or to the central nervous system itself), psychogenic pain (pain not due to past disease or injury or any visible sign of damage inside or outside the nervous system).

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Link: http://www.ninds.nih.gov/disorders/chronic_pain/chronic_pain.htm