The definition of pain, given by the International Association for the Study of Pain, is: 

An unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage. 

Pain levels do not necessarily correspond with the amount of damage to body tissues. In fact there may be no damage at all. 

Pain is divided into two types, acute and chronic pain. Acute pain has a clear purpose. If one accidentally places his/her hand in fire, or cuts it with a knife, he/she would want to know as quickly as possible so as to remove that hand from the source of danger immediately. Even acute pain that lasts a few days is highly beneficial because it protects the injured area from further injury, unnecessary strain, and prevents overloading the injured limb. An injury can heal well and the damage will not worsen.

Chronic pain is defined as pain that lasts more than three months. Chronic pain in itself is classified as a disease in its own right and incurs enormous financial costs worldwide, affecting up to 20% of the adult population.

There are different types of pain, mainly divided into somatic pain (originating from muscles, ligaments, tendons, joints, and bones), visceral pain from internal organs, and neuropathic pain from nerves. However, sometimes there are pains from different sources with pain components from more than one of the above groups. For example, back pain may have a primarily somatic component but also potentially a neuropathic component. Pain can also occur before damage is done or without any damage at all. The pain is also affected by the patient’s emotional experience regarding the pain, especially when the pain interferes with daily activities. Therefore, it is very difficult to objectively measure pain. The emotional aspect has a huge impact not only on the experience of pain but also on the persistence of pain over time. Even when the pain has a structural origin and stems from actual damage to body tissues, a person who is anxious, depressed, focuses on the pain and is afraid to carry on as usual due to the pain, will cause the pain to persist and remain.

The following chapters deal with pain from different sources; the main components are detailed.