The diagram notation used in the ArcObjects object model diagrams is based on the Unified Modeling Language (UML) notation, an industry diagramming standard for object-oriented analysis and design, with some modifications for documenting COM-specific constructs.
The object model diagrams are an important supplement to the information you receive in object browsers. Your development environment, Visual Basic or other, lists all of the classes and members but doesn't show the structure or relationships of those classes. These diagrams complete your understanding of the ArcObjects components.
You can access the object model diagrams from the help system's contents pane by clicking ArcObjects Library Reference, the library of choice, and the Object Model Diagram link; or from their file location, by default <install_location>\DeveloperKit<version>\Diagrams.
Object model diagram key showing the types of ArcObjects and the relationships between them.