Variability refers to how much the numbers in a distribution
differ from each other. The most common measures are presented
in "Measures of Variability." The "variability demo" allows you
to change the standard deviation of a distribution and view a
graph of the changed distribution.

One of the more counter-intuitive facts in introductory
statistics is that the formula for variance when computed in
a population is biased when applied in a sample. The "Estimating
Variance Simulation" shows concretely why this is the case.