The variance of a distribution is a measure of the dispersion, or amount of scatter about the mean. For the population distribution, the variance is roughly the average squared distance between the mean and the individual observations.
The sample variance of a set of data is calculated as the sum of squared deviations of the individual observations from the mean divided by the Degrees of Freedom, (n-1). The square-root of the variance is called the standard deviation.
NIST Statistical Hanbook: Process Characterization - http://www.itl.nist.gov/div898/handbook/eda/section3/eda356.htm#VARIANCE