Nibiru is the (supposedly native) name of the "Twelfth Planet." (The expression "twelfth planet" tends to create some confusion, as we usually accept the existence of only nine planets. An extra planet should therefore be considered to be the tenth. In astrology and ancient astronomy, however, the Sun and the Moon, -as being part of one solar system, together with the 'other' planets- were also called "planets.")
Many people, including quite some scientists, accept the existence of an extra planet (planet X) with an orbit around the Sun.
Sumerian mythology tells the story of Marduk, or Nibiru, a foreign celestial body that entered our solar system. One of its satellites collided with Tiamat, a former planet in between Mars and Jupiter. Part of Tiamat became Earth, the shattered part became the Asteroid Belt. (See Tiamat or Marduk for more information).
According to Z. Sitchin, Nibiru has a 3,600 year orbit around the Sun. Its inhabitants are supposed to be the earliest alien civilization to have walked the Earth. Sitchin refers to them as the Nefilim. These Nefilim would be (co-)responsible for the creation of Mankind, and for the creation of some of the oldest human civilizations, like e.g. the Sumerian. [Note: Earth's Galactic History: Nefilim could also physically refer to the "Lyran giants," who also played a part in the creation of mankind !!!]
Alex Collier claims the Nibiruans, or Anunnaki, came into existence as a new 'tribe,' hundreds of thousands of years ago, when a civilisation from Sirius B and one from the Orion system merged after the marriage of the queen of the Orion group to the king of the Sirian group.
It is interesting to know that e.g. Barbara Hand Clow describes lifetimes on planet Nibiru in her book "the Signet of Atlantis."