Excerpts from the Religionfacts.com

The Mayan calendar was very advanced, and consisted of a solar year of 365 days. It was divided into 18 months of 20 days each, followed by a five-day period that was highly unlucky. There was also a 260-day sacred year (tzolkin), divided into days named by the combination of 13 numbers and 20 names. For longer periods, the Maya identified an elaborate system of periods and cycles of various lengths. In ascending order, these were: kin (day); uinal (20 days); tun (18 uinals/360 days); katun (20 tuns/7,200 days); baktunbaktun (20 katuns/144,000 days), and so on, with the highest cycle being the alautun (23,040,000,000 days). These units were used in the Maya Long Count, which calculated the time elapsed from a zero date set at 3114 BC. In the Postclassical Period, the method of notation was somewhat simplified, and the Long Count katuns end with the name Ahau (Lord), combined with one of 13 numerals; and their names form a Katun Round of 13 katuns. This change makes it difficult to correlate the Mayan count with the Christian calendar, but scholars are fairly confident that the katun 13 Ahau, which seems to have had great significance for the Mayan, ended on November 14, 1539. It has been calculated that the next katun, which the Popul Vuh describes as the catastrophic end of the world, will end on December 21, 2012. Naturally, this has inspired quite a bit of speculation as to what might happen on this date.