The yin-yang is a symbol representing the relationship between yin and yang. It looks something like this:
        .d8P     888888888b.
      .dP      d888888888888b.
    .dP       d888888888888888b.
   .dP       d88888P    Y888888b.
  .dP       d888888      8888888b.
 .dP        8888888      88888888b.
 8P         8888888      8888888888
dP          Y888888b    d8888888888b
88           Y8888888888888888888888
88            Y888888888888888888888
88             Y88888888888888888888
88                Y88888888888888888
88                  Y888888888888888
88                   Y88888888888888
88         d88b       Y8888888888888
9b        d8888b       888888888888P
 Vb       888888       888888888888
 `Vb      98888P       8888888888P'
  `Vb      988P        888888888P'
   `Vb                d88888888P'
    `Vb              d88888888P'
      `Vb           d8888888P'
        `V8b     d88888888P' 

The dark portion represents yin, while the light portion represents yang. The small circle of yin within yang represents how yin is present in the very core of yang. Likewise, the small circle of yang within yin represents how yang is present in the very core of yin.

This symbol is also known as the Tao or the Tai Chi.