On 21 December 2012 the Mayan calender ends, but what does that mean?
Some people think it be like today end at midnight, so what happens at midnight? nothing!
But other think it will be something more significant. In fact the Mayans are not the only ones to think something big is going to happen in 2012.
Some people think it will be the end, wile others think it will be a new beginning.
Theories on what will happen go from natural disasters to the whole planet ascending to a higher dimension.
So lets look at some of the theories.

The Sun.

The sun will be entering a new sunspot cycle in 2012, in fact in may have already started.
When a new cycle starts the sunspots reverse their magnetic polarity. There as already been a sunspot observed with the opposite polarity, but that was short lived.
When a new cycle starts there is increased sunspot activity, which could cause troublesome solar flares. These could knock out power grids and effect communications.
It may also effect the earth's core causing more seismic activity, like earthquakes and volcanic activity.

Magnetic pole shift.

From time to time the Earth reverses it's magnetic poles. We don't really know what cause it to change, but it may have something to do with sunspots or planetary alignments, but a change is well over due.
The poles are beginning to change but slowly at the moment but the change is accelerating.
But what will that mean? Changes in weather, seismic activity, less protection from cosmic radiation and solar flares. We don't really know.

Planetary alignment.

On the 21 December 2012 the solar system will be aligned with the centre of our galaxy the milky way, and like all galaxies it has a black hole at it's centre. It is thought that there will be some radiation coming from this black hole which will effect our pineal gland, and alter our perception of reality. Also the sunspot activity may also effect the pineal gland.
The pineal gland is the minds eye, and the inter phase between the physical and spiritual. The so called truth vibrations may produce enlightenment. or ascension to a higher vibrational state.

Disasters. 

Some people think the world will end because of war pollution, or some man made phenomenon, but I can't see how the Mayans could predict that. 
The Earth could be hit by a asteroid or other space debris. this might be predicted if the Earth passes through a region space that has a lot of space debris.
Some natural disasters might be predicted by knowledge of sun cycles and planetary cycles.

Alien influences

Some people have noticed that there have been a increase in the number of UFO sightings, and think that that could be a precursor to a alien invasion, but most alien encounters seem to be research on Earthlings. They are technically far advanced to us. So if invasion was their plan, why would they need to do any research? They would have just attacked. They seem to need us for some purpose.
Some aliens seem hostile while others seem friendly.
UFO activity increased after we use nuclear weapons. So they seem to have some concern for us. But it might be self interests. They may require us to reproduce, as DNA test on the Star child skull suggests.
I do believe that aliens have manipulated our evolution in some way.
The Sumerian's believed that the Annunaki use genetics to creat slaves from apes. These geneticly modified apes were the first humans.
The Annunaki come from Nibiru, a brown dwarf that orbits the Sun every 3700years. The Annunaki come from one of the planets that orbit Nibiru. 
Because of the elongated orbit Nibiru it only passes near the sun a few days every 3700 years, and some people think that it might be in the year 2012. If it does it might collide with Earth or it's gravitational pull might effect Earth in some adverse way. On the other hand it might pass through with no effect. But if that the case we might have the Annunaki to contend with.
The bible talks about the Nephilim (children of fallen angels). These might be Annunaki human hybrids, and may be related to the Royal bloodline that rules the world ( Illuminati ) The Annunaki might return to see how their children have perverted the system. That why they are hell bent on world domination.
An other type of alien are the pleiadeans these are friendly and want to guide use to a higher plane of existence (ascension). They believe that the earth will pass into a higher dimension due to the "truth vibration" radiating from the centre of the galaxy.

Start of a age.

We have been living in the age of Pisces but in 2012 the age of Aquarius starts. A age last 2150 years, and when it change it is thought to be a bad Oman.