Acupuncture is among the oldest healing practices in the world, having been used in China for thousands of years. Chinese medical theory views all illness and disease as an imbalance in the normal flow of the body's natural energy (qi, pronounced "chee"). This imbalance may occur for myriad reasons - the most common of which being improper diet, physical trauma, and/or genetic factors. The basic foundation of Chinese medicine is that qi flows through the body in a system of channels or meridians, and that each meridian corresponds to an internal organ system. By stimulating a combination of acupuncture points (which are located along the path of these meridians), the free flow of qi is reestablished and a state of harmonious balance may be achieved.
Physically, stimulation of these points with needles results in the release of various chemicals in the skin and muscle tissue (including one called adenosine, the body's natural painkiller) and neurotransmitters (including endorphins, which help regulate hormone levels). Ultimately, effects of acupuncture on the body include improved sleep patterns, decreased inflammation (swelling), increased blood circulation, and improved immune system function.
Acupuncture is generally not painful, but temporary discomfort is possible. You may feel a mild twinge or pinch as the needles is inserted, but the sensation dissipates quickly and you generally don't feel anything while the needle remains inserted.