Wolfram Alpha:

```Data Types
----------

NOMINAL (or CATEGORICAL) basically refers to categorically
discrete data such as name of your school, type of car etc.

ORDINAL refers to quantities that have a natural ordering. The
ranking of favorite sports, the order of people's place in a line,
etc. With ordinal data you cannot state with certainty whether
the intervals between each value are equal. For example, on a 10
point scale, the difference between a 9 and a 10 is not
necessarily the same difference as the difference between a 4 and
a 5.

INTERVAL data is like ordinal data except that intervals between
each value are equally split. The most common example is temperature
in Celsius. The difference between 29 and 30 degrees is the same
magnitude as the difference between 78 and 79.

RATIO data is interval data with a natural zero point. For example,
time is ratio since 0 time is meaningful.  Degrees Kelvin has a 0
point (absolute 0) and the steps in both these scales have the same
degree of magnitude.```