Originally, yin referred to the dark side of a mountain and yang to the lighted side of a mountain. Over the centuries both in general and specifically in Taoism, the concept has been broadened to refer to all complementary opposites. Below is a sample of the attributes that are most widely assigned to yin (on the left) and to yang (on the right):
As you can see, there is a cycle implied. The light part of the figure starts out as virtually nothing and progressively grows, diminishing the dark portion as it goes around, and upon reaching its maximum expression, gives way to the progressive growth of the dark portion of the figure.
An intriguing facet of this symbol is the dot of the opposite color in each side's maximum expression, suggesting that yin is born in the maximum expression of yang, and there is, therefore, no radius within the circle which does not have some of each color in it, no such thing as absolute yin or absolute yang (or absolute anything, therefore) in the material world, the world of manifestation.
It is important to note that yin and yang are relative concepts. One cannot always say that a man is yang and a woman is yin, any more than one can say that a marshmallow is only one or the other (it is yang in color and yin in density). Some times we are more yin or yang in our actions and thoughts regardless of our gender.
For further study of yin and yang, the I Ching (Yi Jing) goes into this concept in greater detail. The hexagrams for Heaven and Earth (often the first two, all yang and all yin) are a good place to start.
"Tao gives birth to one,
One gives birth to two,
Two gives birth to three,
Three gives birth to ten thousand beings.
Ten thousand beings carry yin on their backs and embrace yang in their front,
Blending these two vital breaths to attain harmony."
-- from chapter 42, E. Chen (tr.)
In the Tao Te Ching (Dao De Jing), in the only stanza of the book that gives any explanation of creation, Lao Tzu (Laozi) refers to yin and yang as two vital breaths that are mixed together in all beings: