Normal distribution in terms of standard deviations

Know that, for a normal distribution, values more than three standard deviations from the mean are very unusual; know that approximately 95% of the data lie within two standard deviations of the mean and that 68% (just over two thirds) lie within one standard deviation of the mean.

Be familiar with the conditions which make a normal model suitable.
e.g. that data are continuous, the distribution is symmetrical and bell-shaped, and that mean, median and mode are approximately equal.