A fake planet, invented back in the 1960s, by Zechariah Sitchin for his own story (which has nothing to do with the other one). This fake planet is (in the story) in orbit around the Sun and comes close to Earth every 3700 years (or so). When it does, it cannot harm Earth -- because the people on Nibiru need our help. This story is also one of the "evidence" for the fake theory about humans having been created by alien astronauts.
In Zechariah's story, Nibiru's next passage will be in (or after) the year we call 2085, because it is connected with the Age of Aquarius.
The charlatans decided to combine the two fake planets for the Big 2012 Hoax.
*** End of the Mayan calendar
A lie invented by José Argüelles in 1987 (a book he wrote about calendars). The calendar he described is a real one. However, it is just a day-count calendar that only... counts the days. José picked a round-figure date (there is one every 394 years or so) and claimed that this date would be the end of the calendar. This is the same as the Y2k hoax, where all our computers were going to explode when their internal calendars would "end".
José did claim, back in 1972, that he was a reincarnated Mayan God (not bad for a kid from Minnesota).
However, José did NOT claim that the end of the calendar would be associated with anything bad (and even less with the end of the world). This part was added by the charlatans when they created the Big 2012 Hoax.
*** Planetary Alignment
That part of the hoax is a "copy-and-paste" from a 1974 hoax called The Jupiter Effect. That hoax was about the planetary alignment of March 1982 (a real one, if you want to call that an alignment) and promised us that, among other things, California would slip into the Pacific Ocean on March 10, 1982. I'm still waiting... Maybe we should sue for breach of promise.
There is no special alignment in December 2012.