Yin and yang are intimately related, both in the human body and in the universe at large. In Chinese Medicine there are four basic laws regarding this relationship. These are laws that apply to everything, from intense galaxies to the smallest cells.
First, yin and yang are opposite but relative. An example of this would be summer and winter, where by comparison one looks so hot and the other so cold. But you can also think about the seasons as two extremes of the same thing that gradually change and shift into one another over and over again. They seem opposite but this is really a relative relationship in which a 65 degree day feels warm in winter and cold in summer.
Second, yin and yang are interdependent and can not exist without each other. A great example of this concept is matter and energy. Matter needs energy to exist and energy must be able to manifest itself through matter. In fact, in physics matter and energy are really considered to be just two different manifestations of the same thing. In Chinese medicine they are both manifestations of chi.
Third, yin and yang are mutually consuming. What this means is that yin and yang are in a constant state of dynamic balance, as one increases the other decreases and vice-versa. For example, as the day begins and progresses we see a decrease in the darkness of the night. Then, as the sun set and shadows lengthen, the darkness slowly increases and the day comes to an end.
Finally, is the inter-transformation of yin and yang. If conditions are right and the time is ripe, yin can change into yang and vice-versa. This can happen abruptly like the flip of a coin. A great example of this is a sudden cooling thunderstorm in the afternoon at the height of a scorching hot day.