Nominal
Qualitative scale without meaningful ordering among categories (e.g. gender, ethnic classification)
Ordinal
Qualitative or categorical scale with meaningful ordering or ranking of categories. (e.g. income classification)
Interval
Quantitative scale, but ratios not meaningful nor is there an inherently defined zero. (e.g. temperature)
Ratio
Quantitative scale, ratios are meaningful, inherently defined zero. (e.g. salary, height, distance.)