There are four levels of measurement used in statistics. They are nominal, ordinal, interval, and ratio. This post focuses on the last two levels of measurement of interval and ratio.
Interval level of measurement is used to classify and differentiate between categories based on how different they are. The difference is determined by amount and direction. The difference can also be discrete (finite difference) or continuous (infinite amount of difference). An example of an interval level is temperature. Temperature indicates the difference in hot and cold, you can tell the direction whether it is increasing or decreasing, and it is continuous in that there are an infinite number of potential temperatures.
Ratio level of measurement is the same as interval with the only difference being that it has an absolute zero. One example is weight, it has all the characteristics of a continuous interval variable (there is direction, amount, and infinite amount of difference). The only difference is that nothing can have a negative weight. Temperature on the other hand can go negative (for the sake of illustration please ignore that temperature has an absolute zero).