An important concept in statistics is measures of spread or variation. A basic numerical description of a data set requires both measures of central tendency and spread. For example, say that you know that the average, or mean, income in a certain area is $30,000. Taking this number alone, it could just as easily describe an area where incomes are spread out fairly evenly between $25,000 and $35,000, as an area where most of the incomes are $10,000 and a few very lucky people have an income of $1,000,000. This is where measures of spread come in.
Just as there are several measures (mean, median, mode) used to measure central tendency, there are also several measures of spread, including range, quartiles, variance, and standard deviation.