These represent odd / even, masculine / feminine, positive / negative, day / night. In ancient times yin was the shaded north side of a hill, whereas yang was the sunny south side. Yang governs all things masculine, positive, heaven, hard, moving and living. Yin governs all things feminine, negative, earthly, soft, static and non-living.
The I ching consist of Yin and Yang but these are said to have gave birth to the four primary symbols. An extra line is then added to each of these symbols to create the eight trigrams which relate to the eight directions of space.
Below are the eight trigrams arranged in the Former Heaven Sequence. To see any of the trigram meanings click on the relevant symbol.