Chinese physicians used several different methods to classify traditional Chinese herbs:
The Four Natures (四气 or 四性)
The Five Tastes (五味)
The Meridians (归经)
The Four Natures
This pertains to the degree of yin and yang, ranging from cold (extreme yin), cool, neutral to warm and hot (extreme yang). The patient's internal balance of yin and yang is taken into account when the herbs are selected. For example, medicinal herbs of "hot", yang nature are used when the person is suffering from internal cold that requires to be purged, or when the patient has a general cold constituency. Sometimes an ingredient is added to offset the extreme effect of one herb.
The Five Tastes
The five tastes are pungent, sweet, sour, bitter and salty, each of which their functions and characteristics. For example, pungent herbs are used to generate sweat and to direct and vitalize qi and the blood. Sweet-tasting herbs often tonify or harmonizes bodily systems. Some sweet-tasting herbs also exhibit a bland taste, which helps drains dampness through diuresis. Sour taste most often is astringent or consolidates, while bitter taste dispels heat, purges the bowels and get rids of dampness by drying them out. Salty taste softens hard masses as well as purges and opens the bowels.
The Meridians refer to which organs the herb acts upon. For example, menthol is pungent, cool and is linked with the lungs and the liver. Since the lungs is the organ which protects the body from invasion from cold and influenza, menthol can help purge coldness in the lungs and invading heat toxins caused by hot "wind".