The Mayan calendar is written in the form of a long count calendar which always has an end date, instead of looping like the Gregorian or "western" calendar. The following information from wikipedia will give you a more detailed answer.

Despite the publicity generated by the 2012 date, Susan Milbrath, curator of Latin American Art and Archaeology at the Florida Museum of Natural History, stated that "We [the archaeological community] have no record or knowledge that [the Maya] would think the world would come to an end" in 2012.[12]

"For the ancient Maya, it was a huge celebration to make it to the end of a whole cycle," says Sandra Noble, executive director of the Foundation for the Advancement of Mesoamerican Studies, Inc. in Crystal River, Florida. To render December 21, 2012, as a doomsday or moment of cosmic shifting, she says, is "a complete fabrication and a chance for a lot of people to cash in."[13]

"There will be another cycle," says E. Wyllys Andrews V, director of the Tulane University Middle American Research Institute (MARI). "We know the Maya thought there was one before this, and that implies they were comfortable with the idea of another one after this." [14]

12 ^ Susan Milbrath, Curator of Latin American Art and Archaeology , Florida Museum of Natural History, quoted in USA Today, Wednesday, March 28, 2007, p. 11D.
13 ^ Quoted in USA Today, Wednesday, March 28, 2007, p. 11D.
14 ^ "The Sky Is Not Falling" New Wave, Tulane University, June 25, 2008.
15 ^ See fig. 444 in Wagner (2006, p.283); also Schele and Freidel (1992, p.430).

On that date, the plane of the Earth's solar system aligns with the plane of the Milky Way galaxy. Gravitation radiating outward from the Milky Way's central black hole will become most pronounced, and produce events on Earth negating the further need for a calendar.