Acute pain is an early symptom of injury or illness. It is part of the body's early defence system and helps to ensure our survival. In fact, the sensation of pain is so important that our brain and nervous system have evolved a complex and redundant series of signalling mechanisms and pathways to ensure that the hurt is still felt even if one pathway is somehow damaged.
Chronic pain does not serve any known purpose. As far as is known, it is acute pain that has outlived its usefulness -- the signalling system stuck in the "on" position. This type of pain can be caused by an obvious source of ongoing damage to the body, such as in rheumatoid arthritis. The nerves themselves can be damaged, such as with shingles or carpal tunnel syndrome. However, the most frustrating problems for both patient and physician are those involving a more mysterious dysfunction in the pain sensing or interpretation centers of the spinal cord and brain. Fibromyalgia and chronic migraine fall into this category.