Question

What does it mean if the mean an median of a data set differ greatly?

Answer 1

It usually points to some extreme values in your dataset.

Explanation:

Say you have a dataset where the mean and median are both 100.
If you replace the highest value (say it was 150) by 1000000, this will have a great effect on the mean, but not on the median -- this will still be the value of the 'middle man'.