Normally distributed variables, such as the speed of different automobiles at one spot on the highway, form a bell curve with enough measurements. This bell curve means that most cars of the sample are going roughly the same speed, while the outliers represent faster and slower vehicles.
Other examples of normally distributed variables include IQ measurements, population and test scores. Variables tend to fall between two extremes but are more likely to fall towards the middle of the sample group. In the example of test scores, most students receive an average score on a test, with some students performing better and some worse.