The standard deviation is a measure of dispersion. It is the positive square root of the variance.
An advantage of the standard deviation (as compared to the variance) is that it expresses dispersion in the same units as the original values in the sample or population. For example, the standard deviation of a series of measurements of temperature is measured in degrees; the variance of the same set of values is measured in "degrees squared".
Note: When using the sample standard deviation to estimate the population standard deviation, the divisor (n-1) is typically used instead of (n) to calculate the average. The use of (n-1) allows to reduce the bias of the estimate.