The world can be separated as Yin and Yang. Anything has Yin and Yang, even within a Yin or Yang character itself. Take a second look at the TaiJi graph It is the essential to everything. If you prefer mathematically expression, try this: You can separate 1 into two halves. Each half can be separate into two quarters. each quarter can be separate......

We know there are male and female animals. That is also their Yin and Yang characters. In poetic writings, it is common to substitute female for Yin and male for Yang. 
The days are Yang and nights are Yin. The Sun is Yang and the Moon is Yin. The Winter is Ying and Summer is Yang. The Autumn and Spring are the seasons that Yin and Yang exchange. 

However, do you know the mountain, the plants and our body has Yin and Yang characters?
They do! 
For example, the south of mountain and north of river is Yang. What
is the southern slope of a mountain that is also south of a river?  Then it is on the Yang slope of the Yin river side. 

Yin and Yang is rooted in the fact the world is not homogeneous. There will always be conflicting parts (some rather call it un-similar parts). This is just like the northern and southern slope of a mountain. Even  when the matter of interest is homogeneous, its environment is not.  
The importance is not to memorize Yin and Yang characters of everything, although you have to remember something, but to be able to identify the Yin and Yang characters in any given situation. Here are some basic guide line to get you started, and they could change too.

 

Yin Characters

even number, female, the moon, the earth, cold or cool feeling, night, darkness, soft, stable, closed, come (towards), down (directions), right (side), back, small, loyalty

Yang Characters

odd number, male, the sun, the sky, hot or warm feeling, day, light, hard, moving, opened, go (away), up (direction), left (side), front, large, mercy