Here’s what Wikipedia has to say:

The 2012 phenomenon comprises a range of eschatological beliefs according to which cataclysmic or transformative events will occur on 21 December 2012.This date is regarded as the end-date of a 5125-year-long cycle in the Mesoamerican Long Count calendar.Various astronomical alignments and numerological formulae have been proposed as pertaining to this date, though none have been accepted by mainstream scholarship.

A New Age interpretation of this transition is that the date marks the start of time in which Earth and its inhabitants may undergo a positive physical or spiritual transformation, and that 21 December 2012 may mark the beginning of a new era. Others suggest that the date marks the end of the world or a similar catastrophe. Scenarios suggested for the end of the world include the arrival of the next solar maximum, an interaction between Earth and the black hole at the center of the galaxy, or Earth’s collision with a planet called “Nibiru”.

Scholars from various disciplines have dismissed the idea of such cataclysmic events occurring in 2012. Professional Mayanist scholars state that predictions of impending doom are not found in any of the extant classic Maya accounts, and that the idea that the Long Count calendar “ends” in 2012 misrepresents Maya history and culture, while astronomers have rejected the various proposed doomsday scenarios as pseudoscience, stating that they conflict with simple astronomical observations.

It goes on and on. You can read the whole thing here >

Of course, on the off-chance that the world ends, you’re going to want to know how it is going to end. There are a whole bunch of theories about that. They boil down to:

• We will get sucked into a black hole or, at least, we’ll get close enough to one that its gravitational pull will screw everything up.
• The sun will pass through a “galactic plane” that will cause everything to go haywire
• Solar flares will fry us
• We will smash into a planet called Nibiru

Here are the details, again from Wikipedia:

Other Alignments

Some people have interpreted the galactic alignment apocalyptically, claiming that when it occurs, it will somehow create a combined gravitational effect between the Sun and the supermassive black hole at the center of our galaxy known as Sagittarius A*, thus creating havoc on Earth. Apart from the fact noted above that the “galactic alignment” already happened in 1998, the Sun’s apparent path through the zodiac as seen from Earth does not take it near the true galactic center, but rather several degrees above it,


Even if this were not the case, Sagittarius A* is 30,000 light years from Earth and would have to be more than 6 million times closer to cause any gravitational disruption to Earth’s Solar System. This reading of the alignment was included on the History Channel documentary, Decoding the Past. However, John Major Jenkins has complained that a science fiction writer co-authored the documentary, and he went on to characterize it as “45 minutes of unabashed doomsday hype and the worst kind of inane sensationalism”.

Some believers in a 2012 doomsday have used the term “galactic alignment” to describe a very different phenomenon proposed by some scientists to explain a pattern in mass extinctions supposedly observed in the fossil record.According to this hypothesis, mass extinctions are not random, but recur every 26 million years. To account for this, it suggests that vertical oscillations made by the Sun on its 250-million-year orbit of the galactic center cause it to regularly pass through the galactic plane. When the Sun’s orbit takes it outside the galactic plane which bisects the galactic disc, the influence of the galactic tide is weaker. However, when re-entering the galactic disc as it does every 20–25 million years it comes under the influence of the far stronger “disc tides”, which, according to mathematical models, increase the flux of Oort cloud comets into the inner Solar System by a factor of 4, thus leading to a massive increase in the likelihood of a devastating comet impact.However, this “alignment” takes place over tens of millions of years, and could never be timed to an exact date. Evidence shows that the Sun passed through the plane bisecting the galactic disc only three million years ago and is now moving farther above it.
http://www.youtube.com/watch?v=stDNoJuZRr4

A third suggested alignment is some sort of planetary conjunction occurring on 21 December 2012; however, there will be no conjunction on that date. Multi-planet alignments did occur in both 2000 and 2010, each with no ill result for the Earth.Jupiter is the largest planet in the Solar System; larger than all other planets combined. When Jupiter is near opposition, the difference in gravitational force that the Earth experiences is less than 1% of the force that the Earth feels daily from the Moon

Geomagnetic Reversal

Another idea tied to 2012 involves a geomagnetic reversal often incorrectly referred to as a pole shift by proponents, possibly triggered by a massive solar flare, that would release an energy equal to 100 billion atomic bombs. This belief is supposedly supported by observations that the Earth’s magnetic field is weakening, which could precede a reversal of the north and south magnetic poles, and the arrival of the nextsolar maximum, which is expected sometime around 2012.

Most scientific estimates, however, say that geomagnetic reversals take between 1,000 and 10,000 years to complete, and do not start on any particular date. Furthermore, the U.S. National Oceanic and Atmospheric Administration now predicts that the solar maximum will peak in May 2013, not 2012, and that it will be fairly weak, with a below average number of sunspots. In any case, there is no scientific evidence linking a solar maximum to a geomagnetic reversal, which is driven by forces entirely within the Earth. Instead, a solar maximum would be mostly notable for its effects on satellite and cellular phone communications. David Morrison attributes the rise of the solar storm idea to physicist and science popularizer Michio Kaku, who claimed in an interview with Fox News that a solar peak in 2012 could be disastrous for orbiting satellites

Planet X/Nibiru

Some believers in doomsday in 2012 claim that a planet called Planet X, or Nibiru, will collide with or pass by Earth in that year. This idea, which has appeared in various forms since 1995, initially predicted Doomsday in May 2003, but proponents later abandoned that date after it passed without incident. The idea originated from claims of channeling of alien beings and has been widely ridiculed. Astronomers have calculated that such an object so close to Earth would be visible to anyone looking up at the night sky.For what it’s worth, we don’t find any of those concerns particularly compelling. But we’ve been wrong before.If you’re really worried, you can take advantage of America’s stocked shelves to hoard some food like the Russians. That way, if we do get sucked into a black hole, at least you won’t be hungry

Jesus will return to judge the living and the dead

Bible prophecy: Matthew 24:29-31
Prophecy written: During the first century
Prophecy fulfilled: To be fulfilled

In Matthew 24:29-31, as well as in other places within the New Testament, Jesus revealed about 2000 years ago that he will return in the future to judge the living and the dead. This is a core Christian belief that is reflected in the Apostles' Creed, which is a summary of Christian beliefs that was written many centuries ago. The Apostles' Creed is recited by Christians during Christian worship services. Matthew 24:29-31:

29 "Immediately after the distress of those days " 'the sun will be darkened, and the moon will not give its light; the stars will fall from the sky, and the heavenly bodies will be shaken.' 30 "At that time the sign of the Son of Man will appear in the sky, and all the nations of the earth will mourn. They will see the Son of Man coming on the clouds of the sky, with power and great glory.

31 And he will send his angels with a loud trumpet call, and they will gather his elect from the four winds, from one end of the heavens to the other.
Bible prophecies explained:
End time prophecy from the Bible