Normal Distribution

The normal (or Gaussian) distribution is a continuous probability distribution that acts as a predictor for an astonishing variety of probabilistic phenomenon.  The fact that the equation for the distribution,

contains both π (pi) and e (base of the natural log) makes it particularly interesting to mathematicians.  Both of these constants are highly mysterious and are found frequently in the natural world.

Normal Distribution at Wikipedia

[Note:  In the image above, I placed lines at two standard deviations to either side of the mean.  Don't let them confuse you.]