According to Wikipedia:
Acupuncture is a collection of procedures involving penetration of the skin with needles to stimulate certain points on the body. In its classical form it is a characteristic component of traditional Chinese medicine (TCM). It has been categorized as a complementary health approach. According to traditional Chinese medicine, stimulating specific acupuncture points corrects imbalances in the flow of qi through channels known as meridians.
This holistic approach to medicine attempts to improve physical health conditions as well as affective disorders, and acupuncture has been found by many to increase mental clarity and emotional well-being.
Acupuncture works in conjunction with Qi (the body’s energy), and acupuncture practitioners believe that all illnesses are a result of the natural flow of energy through the body becoming stuck, depleted or weakened (aka: disease, as dis-ease).
Acupuncture rebalances Qi through treatment of specific acupoints related to the symptoms or illness present. While still controversial, clinical trials have proven acupuncture to be effective in treating various medical conditions, and its use has expanded into conventional medicinal practice .