Numbers generally indicate rank order of observations. Some examples may include ranging subjects by height, or order of finish in a race, or relative academic standing in their class. A measure of Average, or Mean, would be meaningless with these data. You could describe Median tendencies of the data, however. This is generally the least-used data type.
Interval data identify subjects by non-relative rank in a scale.
Numbers are assigned to data according to a scale where intervals are constant throughout the entire scale. Some examples may include IQ, Achievement, or Aptitude. Most educational measurement is done using Interval class data. Interval data may be properly described with analyses of statistical Mean, Median, Mode, Standard Deviation and Variance. Ratio data identify subjects by quantification.