Mean and median are two measures of “average.” The mean is the average as we typically think of it: the sum of things divided by the total number of things. The median, in contrast, is literally the number in the middle if we align all the quantities in order. People often use median instead of mean because it is insensitive to extreme outliers which may skew the mean in one direction or another.
For a quick illustration of the difference, I often use the example of income. I choose a plausible average (mean) for the classroom population and review the math. “If Bill Gates walks into the room,” I say, “the average income is now in the billions. The median hasn’t moved, but the mean has gone way up.” So has the Gini coefficient.
Here’s a more realistic and global illustration – the net worth of people in the wealthier countries. The U.S. ranks fourth in mean worth – $301,000 per person…
…but the median is far lower – $45,000, 19th out of the twenty nations shown. (The graph is from Credit Suisse via CNN.)
The U.S. is a wealthy nation compared with others, but “average” Americans, in the way that term is generally understood, are poorer than their counterparts in other countries.