Interval scale definition:
An interval scale has numerical scales in which intervals have the same interpretation throughout, they do not have a "true" zero point, however, and therefore it is not possible to make statements about how many times higher one score is than another. On interval measurement scales, one unit on the scale represents the same magnitude on the trait or characteristic being measured across the whole range of the scale.
But, like many definitions, that doesn't really tell you much. When you are asked to rate your satisfaction with a product or service on a 7 point scale from dissatisfied to satisfied, you are using an interval scale. If you're looking to do calculations about satisfaction levels you can ask a rating question instead.
Interval scale example:
For example, if anxiety were measured on an interval scale, then a difference between a score of 15 and a score of 30 would represent the same difference in anxiety as would a difference between a score of 45 and a score of 60. But they do not have a zero point. For the anxiety scale, it would not be valid to say that a person with a score of 30 was twice as anxious as a person with a score of 15.
Temperature: the same difference exists between 20 °C and 30 °C as between 5°C and 15°C. At the same time, 15°C is not three times as hot as 5°C. A famous example of an interval scale is the Likert scale.