The Eight Principles were first enunciated by Zhang Jie Ben, a famous Chinese Medicine theoretician, in the 17th century. It has since become the most fundamental process for TCM diagnostic evaluation.
It is essentially a way of classifying patterns into logical subgroups that make them easy to diagnose and treat. For instance one of the eight principles is "Cold": it gathers patterns which Western Medicine would describe as "low metabolism" with symptoms like a pale complexion, a weak digestion, hypoglycemia, lowered immunity, anemia, etc.
A beautiful aspect of the Eight Principles is that it pairs very well with the way herbs and formulas are classified in Chinese Medicine. For instance someone with a Cold pattern should take herbal formulas that provide Heat. For instance a typical pattern for women's issue is Cold in the Uterus. Conveniently enough it will get treated with formulas like Wen Qi Hua Shi Tang which belong to the category of "Formulas that warm the middle and dispel Cold" and is composed of herbs with a warm or hot nature.
Click on any principle to read a detailed description of what it means: