The normal distribution (also known as the Gaussian distribution) is a continuous probability distribution. Most processes that are 'in control' conform to a normal distribution, or to some variant of it. Normality is also a condition of many statistical tests.
The normal distribution has a 'bell shape':
The proportion of the process output that falls within a range of values is represented by the area of the curve. Values are:
Between 1 and +1 Standard Deviation 
68.3% (about two thirds) 
Between 2 and +2 Standard Deviation 
95.5% (about 95%) 
Between 3 and +3 Standard Deviation 
99.7% 
All values can be obtained from the Normal Distribution Tables.
