Fine needles are inserted into the skin at certain points on the body, their sites based on a number of theories. Acupuncture is traditionally understood to be a type of ancient Chinese medicine, based on the belief that an energy, or 'life force', flows through the body in channels called meridians. This life force is known as Qi (pronounced 'chee'). Practitioners who adhere to traditional beliefs about acupuncture believe that when Qi cannot flow freely through the body, this can cause illness; acupuncture is a way to restore the flow of Qi, and so restore health.
More modern theories on acupuncture are based on the idea of needling “trigger points” in the muscles. You may be more familiar with these as “knots” or areas of muscle tension or spasm. The belief is that acupuncture stimulates nerves and muscle tissue, and that this may be responsible for the beneficial effects.