The United Nations is a group of every national entity on the planet of Earth, home of the Humans.
As governance of the Earth is currently divided amongst dozens of nations, each with different laws and alliances, the United Nations represents the closest humanity currently gets to the unified leadership groups seen amongst the other species. This is not very close: for the most part the United Nations is a nightmare of bureaucracy and dithering.
However, the United Nations does bear responsibility for the colony of Persephone through the Standing Committee on Extra-Solar Affairs. The Governor of Persephone? and the United Nations Security Force, Persephone are the primary forms of UN control on the colony.