Specialization is a method of dividing entities into their sub-entities based on it's characteristics. This is a done through a IsA relationship between two or more entities.

In a entity-relationship diagram we show this be drawing a relationship with an upside down triangle with the label IsA and then connecting it to the relevant entities. On the upper face (the "top") is connected to the "parent" entity (the one which we specialize from) and then the opposite faces are connected to any entities which are specializations of that entity.