The range, the variance, and the standard deviation are the most common measures of spread or variation.
The range is the length of the smallest interval which contains all the data. It is calculated by subtracting the smallest observation (sample minimum) from the greatest (sample maximum). Alternatively, the range can articulated as simply listing the lowest to highest value (i.e. range of 2 – 10).
The variance is the average difference of each value in the sample from the mean. As, by definition, the sum of the difference of each value from the mean would equal 0, the difference of each value from the mean is squared.
The standard deviation is simply the square root of the variance (see also Standard Deviation (Biostatistics Text))
The standard deviation should not be confused with the Standard Error of the Mean (Biostatistics Text).