A skewed distribution is one which is not symmetrical about the mean, or average. An exponential distribution is one example of a skewed probability distribution.
Skewed distributions can be detected either by looking at a histogram of the data or by comparing the mean and median. The mean is calculated from the data values, while the median is found by locating the data point in the middle of the data set. If the mean and median are equal, the distribution is not skewed. If the mean is greater or less than the median, the distribution is skewed to the right or the left, respectively. A greater difference between mean and median corresponds to a more severely skewed distribution.