Standard deviation is a the statistical measure of how densely located a data set is relative to the mean. A large standard deviation reflects a data set with a large range relative to the mean, while a small standard deviation indicates that most of the data is clustered around the mean. Standard deviation often goes hand in hand with Normal Distribution, also known as the Bell Curve, which has the mean evenly surrounded by all other data points. In such a distribution, two standard deviations on either side of the mean account for 95% of all the data in a set.