Standard Deviation

  The standard deviation is a statistic that tells you how tight or loose the range/spread of data is in relation to the mean or average of the data.  Typically when a data range is clustered tightly the "bell curve" will be tall/steep.  When the data is range is spread far apart the "bell" will appear flat or short and wide.  

Examples

As you can see here; A low standard deviation shows the cluster very close to the mean. The higher the standard deviation the more the values spread apart from the mean.