In such a distribution, the left tail is the part where the extreme values occur, and these values are smaller in magnitude. Since the mean tends to shift towards the extreme values, it is smaller in magnitude. Both the mean and median are lower than the mode, and in most of such cases, the mean will also be lesser than the median.
For a left skewed distribution, the Pearson's Coefficient will be negative, because the mean of such a distribution is lower than its mode. This is why such a distribution is called a negatively skewed distribution.
Read more at Buzzle: http://www.buzzle.com/articles/types-of-skewed-distribution-with-real-life-examples.html