Consider some set of numbers.

The median is the number (not necessarily in the set) where half the numbers are greater than it, and half the numbers are lower than it.

The mode is the number (not necessarily unique) that occurs the most frequently in the list.

The mean is the "average" number (although, again, not necessarily in the set) -- calculated by adding up all the numbers and dividing by the number of things added.

The range is the highest number minus the lowest number.