Ok, supposedly, the Mayan Calendar ends on that date. Because the Mayan Calendar has supposedly predicted significant weather-based events or some shit, people think that "OMGTEHWORLDITPHAILS!," rather than the slightly more rational "Well, maybe the Mayans got bored, or, you know, butchered by disease-ridden Spaniards or something." And so, because of this tiny theory, you know have the Bible Code folks jumping on the wagon, Roland Emmerich making yet another big-budget movie of questionable quality, and a bunch of people running around like nutters.
I just want to say the Mayans never thought the world was going to end. The end of a Great Cycle (this is the fourth Great Cycle) was a time of celebration. The only reason 2012 is different is that it falls on the Winter Solstices and the Sun will line up with the center of the Milky Way. Not that exciting.