Yin is a Chinese term that literally means "shady." Yang is a Chinese term that means "sunny." It's pretty basic. Yin is feminine and Yang is masculine. The Yin/Yang represents opposite yet complementary parts. They are most often represented in the western culture by the Yin/Yang symbol. The dark half represents Yin and the light half represents Yang. So in essence, it reduces the world to black and white.
To me the Yin/Yang symbol has always looked like two sperm chasing each other's tails. It also reminds me of my Astrological symbol, Pisces, which is symbolized by two fish chasing their tails. I could go down all kinds of philosophical Taoist roads in talking about Yin and Yang, but I think it just boils down to two opposites chasing each other's tails.
Oh we could use more eloquent terms like duality or dichotomy when we talk about Yin/Yang. But I truly do think it is the delicate dance between opposite natures that is the building block for our world(s). I don't think anything is actually all Yin or all Yang. To me the goal is to balance the two as they chase each other's tails within you. But then again, I have a tendency to oversimplify things.
It isn't always easy to see opposites as complementary. We are taught to view good and evil, happy and sad, hardworking and lazy, and so on. And some religions even go so far as to justify all of the evil in the world by blaming it on free will. They say polar opposites are neccesary in order to provide us with a choice. The concept of Yin/Yang short circuits that concept by mergering all of those opposites into balanced parts of a whole. You don't have to choose Yin or Yang. You just have to accept both. It kind of kicks the concept of sin in the butt.
Again, I have a tendency to oversimplify.