Five phases or five elements?
The five elements or five phases are a system of correspondences and relationships in Chinese medicine theory. Each organ has a five phase correspondence and a relationship with each other organ according to two cycles, the generating (sheng) and controlling (ke) cycles. The five phases are Fire, Earth, Metal, Water & Wood.
Each acupuncture channel also has a five element correspondence, and each channel has five acu-points that are ascribed the different phases. Different kinds of disease and symptom also have five phase correspondances. These help explain the course of disease, and guide the practitioner to the appropriate treatment strategy.
There are different ways and systems of how to combine and use the five element points in practice. The Nan Jing, (classic of difficulties, ~200AD) outlines one, which forms the basis of much Japanese acupuncture theory. There is also a system calling itself five element acupuncture which is different and has another way of combining the acupuncture points.
The Five Phases
Sometimes translated as five elements, wuxing is more correctly translated as five phases because they are like yin and yang, dynamic and interacting relationships. The character xing signifies movement and change. The reason it was translated as element is becuase of a jesuit missionary in China in the 17th century who saw a similarity with the ancient Greek idea of four elements.
These associations bring in another level of organ correspondences and another way of approaching diagnosis and treatment and are quite key to the understanding of pathological mechanisms according to Traditional Chinese Medicine.