simplystatistics:

The interesting thing is that even though we only estimate that Obama leads by about 0.5%, he wins 68% of the simulated elections…
He never gets more than 54% or so and never less than 47% or so. So it is always a reasonably close election. Silver’s calculations are obviously more complicated, but the basic idea of simulating elections is the same.

An excellent, simple explanation of Nate Silver’s black magic, and what a “75% chance of winning” actually means.