The two most important types are Entity and PlaybackItem. A PlaybackItem can have a property that is an entity.

Head over to the API Explorer to see the different types of Entities and PlaybackItems available.  The documentation will also have a description to get a better understanding for when/how you would use these types in your application.

Entity types can be viewed with the entity metadata query, whereas PlaybackItem types can be viewed with the playback current query that returns a type of PlaybackItem.