In statistics one has 4 basic data types: nominal, ordinal, ratio, and interval.
Nominal basically refers to categorically discrete data such as name of your school, type of car you drive or name of a book. This one is easy to remember because nominal sounds like name (they have the same Latin root).
Ordinal refers to quantities that have a natural ordering. The ranking of favorite sports, the order of people's place in a line, the order of runners finishing a race or more often the choice on a rating scale from 1 to 5. With ordinal data you cannot state with certainty whether the intervals between each value are equal. For example, we often using rating scales (Likert questions). On a 10 point scale, the difference between a 9 and a 10 is not necessarily the same difference as the difference between a 6 and a 7. This is also an easy one to remember, ordinal sounds like order.
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.
Interval data is like ordinal except we can say the intervals between each value are equally split. The most common example is temperature in degrees Fahrenheit. The difference between 29 and 30 degrees is the same magnitude as the difference between 78 and 79 (although I know I prefer the latter). With attitudinal scales and the Likert questions you usually see on a survey, these are rarely interval, although many points on the scale likely are of equal intervals.
Now having stated all of above, I am confused with clarification of the following:
A list of metals and their melting points is of what type? First is nominal, and second is interval?
Best Answer
The nominal-ordinal-ratio-interval scheme is not a scheme for "data." It is a scheme for ${\it scales}$. A list of ordered pairs like (lead, 700 K) does not belong to a scale. So we don't ask what type of scale it is. That is like asking whether (Mars, Jupiter) is a terrestrial planet or a gas giant.
Here is the original paper that introduced this typology: http://personal.stevens.edu/~ysakamot/719/week3/Stevens_Measurement.pdf
If you were to consider only the names of the metals, that would be nominal. If you were to consider only the melting points (in Kelvin), that would be ratio. Both of those are scales and so can be classified according to a typology of scales.