Elijah is right. The wold won't end til the sun supernovas. What most people mean when they say the end of the world is the end of mankind. There are several events that could occur any time now that would come close to doing that.
Massive volcanic eruptions, which I put a high level of probablility on in the next 100 years.
The Earth's magnetic poles shifting. Which seems to occur about every 600,000 years and is overdue, and has a very high probablility level
The Earth's axis shifting, which is possible, but doesn't have a high probability.
Each of those events could, if severe enough, end mankind. But let me put it this way. I'm not terribly worried.
As far as 2012, we simply don't know enough about the Mayans to know what they were predicting accurately. We only know they predicted a major event at this time.