Okay... Don't listen to anyone else because most of them are dumbasses. The spiral (Yin-Yang) is most commonly Good and Evil (White and Black), but it can be among other things as long at it's opposing the other.
Dan: Yin Yang Is spelled Y-I-N-G Y-A-N-G. Cory: No, dumbass, read a book, it's spelled Y-I-N Y-A-N-G.
The other definition here claiming that yin yang refers to anus is correct, and older people in certain regions of the US know this very well. This site is for "slang" usages, not the formal dictionary definition which would refer to the Chinese symbol. When a person says something like, "Wow, this store has widgets up the yin yang", it does not mean plethora. Rather, it's a substitution for the word ass. And in a sexual context, yin yang does indeed refer to the anus. "Taking it up the yin yang" is probably the most used phrase, although it's now somewhat antiquated, and it means taking it up the anus, or male/female part which is common to men and women, which is how the word was derived since the Chinese symbol represents the male/female balance.
Yin/Yang of the taoist tradition of ancient China is meant to represent the concept of polar opposites. For instance Yin is represented as darkness and yang as light. Yin as rest and Yang as activity. Often symbolized as the separation of the primordial Wu Qi- the black and white Yin Yang symbol most often seen in East and West culture. It is the most important concept in Traditional Chinese Medicine where the doctor heals the patient according to their imbalances in relation to Yin Yang energy within the meridians or channels.