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They observe and record its values.

This type of variable is sometimes called "nominal" as in name.  The values are categories that do not have a numerical meaning. Examples are: male / female, urban / suburban / rural.

A quantitative variable has a numerical scale in which the numbers have value than can be described as more or less. Examples are height and weight.

Ordinal, interval, and ratio.

The measures of a quantitative variable are ordinal when we know the rank order. For an ordinal scale, we only know the order. The distances between levels are unknown. We know that one level is  higher than another level, but not by how much. Star ratings for hotels is an example of a ordinal scale.

An interval scale has the properties of an ordinal scale, but the numbers on an interval scale also have equal distances or intervals between them. There is no true zero on an interval scale which limits what we can say about values coming from the scale. Temperature in degrees Celsius is an interval scale. We can say that the interval between 5 and 10 degrees and 15 and 20 degrees is the same. We cannot say that 0 degrees is "no temperature."

The ratio scale has the ordinal and interval properties of order and equal intervals. The value of 0 on the scale means there is no quality. Income is an example of a ratio scale. Higher numbers are greater income; intervals are equal all along the scale; and 0 means no income.