The links here lead to more detailed information, but the basic summary of the 2012 issue (as I see it) is:
1) The solar system alignes with the Galactic Center on this date (Dec 21,2012).
This has not happened for 25,000 years (5 5000 year cycles of the Mayan calendar, which is all based on cosmic alignments)
2) The center is a huge black hole that keeps our galaxy spinning around it, similar to how our sun keeps planets around it.
3) This black hole has a TON of radiation - similar to what you may get from a supernova.
4) The radiation hits the earth - and as a side effect creates low level cloud cover
(the radiation excites particles near the earth's surface, that allow water vapor to condense around them, when otherwise there would be nothing for the water vapor to 'grab' onto...this grabbing makes clouds near the earth surface)
5) Ontop of this, the radiation promotes mutations in life forms on earth - all life forms.
6) The low clouds reflect light and heat (white reflects best) and we get the start of an iceage.
7) While most species die in the cold, high levels of mutations accelerate evolution. The ability to adapt is accelerated, and life changes rapidly.
8) With most everything dead, the rest mutated, and (eventually) the planet warming again, life goes on....
9) But it is NOT life as we know it... Thus the debate over 2012 being the start of the end, or a new beginning...
Hope this helps! Feel free to explore more, or just watch the clock!!