Today's Zits comic strip pretty well sums it up:
It has to do with the Long Count calendar, specifically. No Doomsday in 2012 points out that the current argument made by archaelogists and mythologists is that the Mayans saw this end as the beginning of an age of enlightenment, not a doomsday scenario. However, it is debated whether the calendar will reset then (on a base 13, or 126.96.36.199.0.0) or approximately 8000 A.D./C.E. (on a base 20, or 188.8.131.52.0.0). The Mayan numeral system is based on 13 and 20.