This Taoist symbol represents balance. Dark and light, ebb and flow, good and evil, female and male, pretty much any opposite forces may be used to form a yin yang but it is traditionally a circle with equal curving teardrops of black and white with circles of the opposite color within those teardrops. These circles may symbolize that there is one force within the other and vice versa. Oddly enough in the traditional description of a Shinto dragon there are many more yang scales than yin, which is said to represent that it actually takes much more good to balance out a little evil, or much more peace to balance out chaos....I think that you get the idea.
Others feel that the entire good vs evil idea is a modern or Western ideology. All things are nature in one form or another, which is neither good nor evil. This idea does seem to be at odds with the negative/positive attributes still associated with the halves of the symbol but these may also refer to subtraction/addition rather than evil/good.
There's a little more information and designs here.