Simple measures of spread

Range
Difference between maximum and minimum values
Inter-quartile range
The middle half of the values are within an interval of this length

These are (relatively) easy to understand and explain to others, but neither are commonly used.

Standard deviation

The standard deviation is a 'typical' distance of values from the sample mean.

The standard deviation is denoted by the letter s and is defined by:

The numerator, , depends on the distances of the values to the mean, so it will be small if the values are all close to the mean and big if they are far from the mean.

Variance

The square of the standard deviation, s2, is called the sample variance. Variances are sometimes reported and used but standard deviations are easier to interpret since they have the same units as the original data (e.g. kilograms or dollars).