Standard deviation
The standard deviation σ of a series of values xi is calculated as
$$ \sigma\ =\ \sqr{\frac{1}{N} \sum (x_i - x_{avg})^2} $$
with x_avg the average of all the x_i, and N the total number of values.
Its squared value σ2 is an estimator of the variance.
See http://en.wikipedia.org/wiki/Standard_deviation and http://en.wikipedia.org/wiki/Variance