Standard Deviation 

The concept of standard deviation was first introduced by Karl Pearson in 1893. The standard deviation is the most important and the popular measure of dispersion. Unlike the mean deviation which may be computed from any measure of central tendency standard deviation is always computed from arithmetic mean, While taking deviations of the values from mean,  the standard  signs are not ignored, Theses deviations  are squared up and totalled. The sum of the squares  of deviations is divided by  number of items. The square root is taken of the average of deviations is divided by  from mean to obtain the values  variance  standard deviation is also called mean error or mean square error or root mean square  deviation. thus the standard deviation is the square root of the arithmetic mean of the squared of all deviations, deviations being measured   from the arithmetic mean of the observations. It is represented by Greek letter sigma small.

Coefficient of standard Deviation:

standard deviation is an absolute measure where comparison of variability in two or more series is required to be made relative measure of standard deviation is captured. It is called coefficient of standard deviation. It is calculated by dividing standard deviation by the mean of the distribution.