Original: For example, a "subscriber" role type describes the property common to instances of the class "Person" when they participate in a "subscribes-to" relationship with the class "Magazine".
Simplified: In this example, there are three classes: 1) a class called "car", 2) a class called "pick-up truck", and 3) a class called "vehicle". Although there is only one class called "car", there could be many objects that are created from the class called "car".