The five elements
These are the phases through which chi moves.
You can think of the elements acting on each other as an advanced game of "Rock, Paper, Scissors" -- there are two cycles, productive and destructive.
This symbol is one of the most readily recognizable of Chinese culture. The black and white swooshes are connected, with a dot of the opposite color in each. The concept behind the yin and yang are that they are opposite states of chi (energy). One cannot exist without the other. Yin (black) is associated with femininity, matter, nighttime, coldness, passivity and softness, and yang (white) is about masculinity, spirit, daytime, warmth, activity and hardness. If you didn't have night, you couldn't understand what day is, so neither can exist on its own. In the world of feng shui, the yin and yang must be balanced.
While chi, and thus yin and yang, are constantly in motion, the symbol is traditionally displayed with the yang on top, under the assumption that heat rises.