Standard deviation is a measure of dispersion. It is defined as the square root of the variance. It measures how widely spread the values in a data set are. If many data points are close to the mean, then the standard deviation is small; if many data points are far from the mean, then the standard deviation is large. In a loose sense, the standard deviation tells us how far from the mean the data points tend to be. The standard deviation has the same units as the data points themselves.
When applied to time series, standard deviation becomes a measure of volatility.
See also: http://www.childrensmercy.org/stats/definitions/stdev.htm
http://www.quickmba.com/stats/standard-deviation/
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