Yin and Yang

We did go for a walk around the lake the other day.

Not today, on account of it's as cold as witches somethings or others, and we ain't going nowhere.  Now, where was we. 

Oh, yes - a walk around Playford Lake, and we did come across these two boys sitting on a bench together and they was smoking something which smelt a bit peculiar, but ours is not to reason why and they did seem very mellow.  Anyway, one of them did say we was yin and yang. 

So - the question is, what's a yin and what's a yang, and which one is which?  Bother, that's three questions.  But you do know what we do mean.