Nibiru ( In Sumer language) or Marduk ( in Babylonian ) is a planet which enters to the solar system in each 3600 years. Once it had entered accordign to the Sumerians one of its moons has collided with the planet called Tiamat and as a result of this an asteroid belt and the Earth appeared as new members of the solar system.
In modern terms Nibiru is also called as Planet X. It is claimed that it will enter to the solar system again in the near future and pass by Earth and at the end it will terminate the civilization by creating earthquakes, tsunamis and polar shift. Many articles and conspiracy theories have been written about this. Some argued the the comet Elenin and Comet ISON to be the Nibiru and will cause above mentioned disasters on Earth. Sout Pole Telescope according to some is believed to be manufactured to observe Nibiru and it believed that the Nibiru is already in our solar system.
According to the ancient Sumerians, Nibiru is the homeland giant planet of the aliens called as Anunnaki. It is a red giant planet and Anunnaki living on it came to the Earth to collect gold mine in order to save their own planet's atmosphere.
Check the Books about Nibiru here