Apart from dark teas, tea is never fermented. The tea leaves contain enzymes that react with the oxygen in the air as soon as they are picked from the tea tree. Thus, tea leaves turn dark due to an enzymatic oxidation process which breaks down the chlorophyll, the green pigments of the leaves. This is also often referred to as browning and is the exact same process a banana or an apple undergoes when you slice it open and leave it for a while.
The process of fermentation always requires the activity of microorganisms, for exampe certain bacteria or fungi, to bring about a desirable change to food or beverages. The use of such microoganisms only appears in the production of dark teas, also known as post-fermented teas.