Thanks to the early books of John Major Jenkins, a lot of people believe there will be a unique "Galactic Alignment" on Dec 21, 2012, and that it will have a profound effect on human consciousness, or the gravitational effect will cause a cataclysm on Earth..
The alignment is between Earth, the Sun and the Galactic Plane/Equator. They are never truly in exact alignment, but this is the closest they get in each 25,000 year cycle known as precession.
These days JMJ describes the alignment as being "in the era of 2012". This is because the closest we came to alignment was in 1998. Because of this, we can quite safely state that the alignment had no immediate effect on humans or Earth. All that really leaves is a long-term effect, and you'd be best describing that as astrology.
So, if you are into astrology, there may be something in it. Otherwise, it cannot cause anything to happen to us or our planet.
Note 1: There is nothing special about the alignment of our planets in 2012. There was a significant alignment on 5/5/2000, but as many experts predicted, nothing happened.
Note 2: It could be argued that the Mayans made their best prediction of when the alignment would occur, and they used that date (2012) to end their Long Count calendar. If this is the case, although they were 14 years out, this was significantly better than our scientists could have managed a few decades ago. The end of the Mayan Long Count calendar could be nothing more than when the Mayans predicted a galactic alignment that they considered to be ominous.