The Mayan calendar ends abruptly in 2012, thus this year is predicted to be the end of the world according to several prophecies.
There are actually three Mayan calendar systems, the 365-day Solar year, the 260-day Ritual year, and the 5,128 years of the World Time calendar. The Haab or Solar year was broken down into an 18 month plus five days cycle. 18 months of 20 days and the 5 soulless days which were thought to be of ill omen, kind of like 5 days of Friday the 13th. The Tzolkin (zol-KEEN) or Sacred Round was the 260-day ritual calendar was broken down by days, not months. This religious calendar was the basis on how the people, singly and collectively, went on with their day-to-day lives according to destiny.
Their long count calendar, based roughly on a 5,000 year cycle, is a massive and highly accurate list of astronomical events and seasonal occurences such as solstices and equinoxes.
The calendar simply ends on Dec. 21, 2012 (the winter solstice), and the Mayans did not offer a calendar addendum to specify anything much happening after that.
Like the Aztecs after them, the Mayan's made reference to the world being created and destroyed several times. They believed they were living in the 5th age of the sun, and that 4 previous ages had existed. The Mayans believed that there had been great death and destruction at the end of each of the previous worlds. The Aztec description of the end of the 4 suns is as follows:
1st sun - destroyed by water
2nd sun - destroyed by wind
3rd sun - destroyed by fire
4th sun - destroyed by rain of fire and blood.
(there is some controversy concerning the correct ordering).