When you are working with the probability that an event could happen, that is called theoretical probability.
For example, when rolling a typical sixsided number cube, there is only one "6" on the cube and an equal chance of any number landing face up.
There are other types of probability. Sometimes, a simulation or a series of events has already occurred, and there is data that can be used to make a prediction. If you are estimating probability based on data that has been collected, it is called experimental probability.
For example, if a dart is randomly thrown at a dartboard 40 times, data can be collected based on the numbered sector in which the dart lands.
Dartboard Data


Range of Numbers

Number of Dart Landing

1–5

11

6–10

9

11–15

8

16–20

12

Working with both theoretical and experimental probability is important, since frequently in the real world, what should happen, and what really does happen, are quite different!