From ACT Wiki
Statistics - measures of spread.
Standard deviation measures the spread of data around their mean.
The standard deviation is the square root of the variance.
Standard deviation is used widely as a measure of risk, because it is relatively easy to calculate, and to compare and combine with the standard deviations of other variables.
Its order of magnitude is the difference, ignoring the sign, between the mean and a randomly chosen item in the population.
Standard deviation is a similar concept to mean deviation, but it is calculated on a more refined basis.