In statistics, mean and median both provide an idea of where the “middle” of a
sample is.

The mean (or average) is the sum of all scores divided by the
number of scores. For instance the sum of individual ages of persons in a group
divided by the number of persons in the group, gives the average age.

The median is the number in a range of scores that falls
exactly in the middle so that 50% of the cases are above or below.

Source: GreenFacts

More:

The example below shows that mean and the median do not necessarily provide
the same result: