Skewness
When the mean, median, and mode are unequal, the normal curve can become skewed in either a negative or positive direction depending on their values in relation to each other. In each of the above panels, the dotted line shows how a normal distribution would be shaped (if the mean and median were equal). In the left panel, showing negative skew, the mean is less than the median; in the right panel, showing positive skew, the mean is greater than the median.
Source
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:
"Statistics Ground Zero/Descriptive Statistics."
http://en.wikibooks.org/wiki/Statistics_Ground_Zero/Descriptive_Statistics%23Shape
Wikibooks
CC BY-SA 3.0.