http://www.levity.com/eschaton/Why2012.html

To sum it up if you're illiterate, the Mayans were extremely advanced astrologists and predicted a point in time at the end of their calendar when our sun will reach the direct center of the galaxy on Winter Solstice.

Supposedly this will correspond with the ascension of the human race into a new world age, one less towards ignorance and hatred, and more towards enlightenment and cooperation.