Yin-Yang release is the ability to create anything out of nothing. Izanagi is Yin-Yang release, and allows you to create an entire scenario, a fantasy, out of absolutely nothing. You died, but you rewrite fate to make it so that you never died. Usually Yin-Yang Release has very limited parameters and relates mostly to the Rikkudo Sennin. However a lot of people like to have their own snowflake version.

Yin is the mind creating a shape, but never birthing it into a tangible form. That's why Genjutsu are Yin-Release, they're constructs that never actually interact with the physical world.

Yang enhances physical, but can never be more than something applied to something else. Like increasing the size of an object or limb. With Yin, it can give form to something that doesn't exist in this realm.

That's the best interpretation I can offer. There are varying ways it's used. Such as how the Rikkudo forms of Obito and Madara can suppress the effects of Ninjutsu such as Edo Tensei. Though you could argue that it's the reversed effects of Yin-Yang, where it can de-construct anything.

All in all: Yin-Yang can be seen as 'doing whatever the fuck you want', but often defined in very clear and restricting parameters, which prevents it from being omnipotence. Some people think they are clever by making their char Yin-Yang, but in reality it's just the mother of  all open-ended shit.

Edit: Odin is right that chakra uses the principles of Yin-Yang, though I'm speaking of actually manipulating the concept of Yin-Yang (which is called Yin-Yang Release, or Omnyouton). Almost all ninjutsu follow the principle of Yin and Yang mending together to create a specific form, often limited and defined by the nature release. Convoluted, I know.