Brief description of nominal, ordinal, interval, and ratio measurement level
Nominal: categorical, with no ranking in the categories. For instance, religion.
Ordinal: categorical, with ranked categories. For instance, socio-economic status.
Interval: quantitative, where the difference between scores is meaningful. For instance, temperature, measured in celcius degrees. Also, sometimes Likert scales are considered interval scales.
Ratio: quantitative, where the difference between scores is meaningful, as well as the zero value. For instance, body length.