A median is the point that divides a series of numbers so that half are on one side, half on the other.
In the series 1, 4, 8, 20, 24, 27, 42, the median is 20.
The mean is the average of a series, found by dividing the sum by the number of elements.
In the series above, the mean (or average) is 18, or 126 divided by 7.