A frequency distribution with the majority of occurrences in the centre and tapering off on either side. Normal curves are sometimes known as bell curves. When all measures of central tendency occur at the highest point in the curve, it is said to be a "genuine" normal curve.
In psychology, the normal curve is a powerful, recurring phenomena. A frequency distribution of people's height is an example of a normal distribution. The majority of people would be of medium height, with exceptions on either side.