As mentioned in this writer’s articles here and here, it is difficult to understand Yin and Yang independently. The interaction between them is known as the Tai Chi (“Great Ultimate”), represented by the Symbol "Tai Chi Tu", called by most Westerners the “Yin-Yang Symbol”. The Tai Chi Tu is said to have been invented by Fu Hsi, the semi-Divine First Emperor.
This design is a circle divided in half, into individual sections in the form of curlicues. The left-hand, upward-moving curlicue is white, representing Yang, and the right, down-ward moving one is black, symbolizes Yin. Both halves of the design contain a small circle of the opposite color. It is important to note that, in Chinese Culture, since the most distant Antiquity, Left is honored over Right, which is the exact opposite of Western (European) usage. However, even in the West hot faucets are on the left, and cold on the right. The combat method called "Tai Chi" is of course, related to all of this.
The concepts of Yin, Yang, and the Tai Chi are very Ancient. Perhaps you might remember the referent to this in Chapter Thirty-One of Lao Tze’s Tao Te Ching?
31.1 Finely sharpened weapons are hated instruments of evil. Therefore, those who possess the Road turn away from them.
31.2 In peace leaders favor the creative left. In war they favor the destructive right.
31.3 Weapons are instruments of evil, not the instruments of a good Monarch. When the Monarch cannot avoid using them, calm restraint must be applied as the best Principle.
31.4 Even in victory there is no glory. Those who celebrate victory delight in slaughter. Those who delight in slaughter will not be successful leaders.
31.5 At a feast, the high seat is on the left; at a funeral, on the right. The Lieutenant General leads at the left wing, Senior General, the right wing; it is as if the battle were a rite of Mourning!
31.6 The killing of many should be mourned. Let us weep, for the slaughter of a multitude. This is why a victory must be observed like a funeral.
Yin and Yang are viewed as in constant alternation. Nothing in this Universe can be entirely Yin or Yang. As depicted in the Tai Chi Diagram, both contain the Seed of one another.
Lao Tze mentions Yang and Yin by Name only once, Chapter Forty-Two.
42.3 People despise the Thought of being orphaned, needy, and unworthy, yet Monarchs and Nobles call themselves such.
42.4 Thus increase brings decrease to some, and decrease brings increase to others.
42.5 What another has taught, I also teach: "A violent man will not die a natural death." I will make of this the father of my teaching.
It might be interesting to note, that both these Chapters of the Tao Te Ching appeal to the restoration of peace in the World. It would involve balancing Yang and Yin, wouldn’t it?