Scales used to express measurements. Usually ratio, interval, ordinal or nominal types of scales are recognized. Dichotomous and count types are sometimes seen as their own. Other classifications exist. [For "scale" as a sum of individual items in psychometry - see also tag 'psychometrics'.]

See the original article:

S. S. Stevens, On the Theory of Scales of Measurement.

For modern criticism, a good place to begin is:

Paul Velleman and Leland Wilkinson, Nominal, Ordinal, Interval, and Ratio Typologies are Misleading. http://www.cs.uic.edu/~wilkinson/Publications/stevens.pdf