Median is a better measure of central tendency when there are outliers in the data. The mean is vulnerable to outliers -- that is to say that the mean can be skewed in the direction of the outlier. So for your income example, let's imagine that you have a sample of 50 individuals and their yearly salary is reported. If 49 people have a yearly salary between $50,000 and $60,000 and then Bill Gates (who happens to be sampled) has a yearly salary of 1.1 billion....your mean is going to be heavily skewed upwards to reflect the outlying salary...thus a median will give you a more reliable measure of central tendency as it remains unaffected by outliers.

hope this clears things up.