Elenin is a real comet, rather small (and probably breaking apart as we speak), passing very far from Earth. This kind of thing happens at least a dozen times every year. Comet Elenin is, at best, 4 km across, so that there is no way it can affect planets that are a gazillion times the mass of the comet.
[comets, though, have always been popular with charlatans, and it is known that they can be used quite easily to scare the unwashed masses]
Nibiru is a fake planet. It was invented in the 1960s by Zechariah Sitchin for a fake theory he had: we (humans) were created by people from planet Nibiru, to be their slaves and dig for gold, which they need for their survival. In Zechariah's story, the next passage of Nibiru is "scheduled" for the year 2085. When Nibiru passes close to us, it cannot harm us (they need our help for the gold).
Planet-X is a different fake planet. It was invented by Nancy Leider who claims that she was abducted by aliens. These aliens would have told her about a rogue planet that was going to run across our solar system, passing too close to Earth. They gave her the task to warn us so that we could prepare. Even with preparation, over 90% of us would die: global tsunamis, earthquakes, volcanoes, pole flips...
The aliens even gave Nancy the date: May 13, 2003.
"Friends" of Nancy used her story to scare gullible people, then sell them books on how to survive.
After May 2003, they needed a new hoax, so they re-loaded their Planet-X story, use another lie for the date (the end of the Mayan calendar is a lie invented by José Argüelles in 1987), and "improved" their Planet-X by stealing ideas from Zechariah's fake planet.
The rest of the Big 2012 Hoax are fake predictions taken from other hoaxes that were not even meant for 2012 when they were written (for example, the planetary alignment doomsday predictions are from a hoax called The Jupiter Effect, written in 1974, for the real planetary alignment of March 1982).