Acupuncture is the insertion of very fine, filiform needles through the skin at specific points on the body with the intention of manipulating Qi. The filiform needles are solid, as opposed to the hollow hypodermic needles most people are familiar with, and are usually made of stainless steel, but can also be gold or silver.
How does Qi flow through the body?
Acupuncture is based upon the jing luo channel network theory of the circulation of Qi. Although Qi permeates every part of the body, it tends to collect and travel along channels called "jing luo." These are the so-called "meridians" of acupuncture.
The jing luo channel system connects all aspects of the body together into one network of energetic communication.
Just as water flowing through a landscape tends to seek the path of least resistance, so Qi flows through the body. The flow of Qi follows the folds and creases of the body's landscape. It follows the divisions between muscles and the clefts between muscles and bones, collecting in the small hollows and depressions of the body to form pools of Qi.
These "pools of Qi" are places where Qi is concentrated and more accessible. They are the acupuncture points, where Qi can be accessed and manipulated through the use of finger pressure (acupressure), massage techniques (tui na; literally "pinch and pull"), dermal friction (gua sha), cupping, moxibustion (a form of heat therapy), and, of course, acupuncture.