Scales of measurement are the ways in which we approach and measure variables.
Nominal scales use categories, but they cannot rank the variables, e.g. into nationality or religion. Nominal scales have frequency, but we do not calculate mean or median.
Ordinal scales can rank categories, but they do not describe the degree of difference between objects. A good example of ordinal scales is military ranks.
Interval scales also rank categories as well as describing exact differences between them. Interval scales do not have a zero value. Intelligence scales are an example of interval scales.
Ratio scales are the same as interval scales, but have a non-arbitrary zero value, e.g. Kelvin scale.