What is the difference between the mean and the median of the data set?

{22, 8, 10 ,18 ,12, 20}

The "median" is either the central term of an ordered set of odd number of elements or the average of the two central terms in an even number of elements.

First order element from least to greatest:

{8,10,12,18,20,22}

So the median is (12+18)/2=15

The "mean" is jus the normal arithmetic average that you are probably quite accumstomed to.  It is the sum of elements divided by the number of elements.

mean=(8+10+12+18+20+22)/6=15

So in this case, there is no difference between the median and the mean.