Numerical Measures of Central Tendency and Variability
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.