Human Longevity
When comparison of human lifespans is done, most people live beyond their middle age, or even older. Thus, the maximum frequency is of such people, which takes up the right side of the distribution, which is the side with higher age values. However, some individuals lose their lives younger, and some even at a very young age. These individuals take up the lowest values, i.e., towards the left side or the negative side of the distribution, making the tail longer at this part.
To sum it up, a positive skew distribution is one in which there are many values of a low magnitude and a few values of extremely high magnitude, while a negative skew distribution is one in which there are many values of a high magnitude with a few values of very low magnitude.
Read more at Buzzle: http://www.buzzle.com/articles/types-of-skewed-distribution-with-real-life-examples.html