The Unified Modeling Language (UML), is the most commonly used language for object-oriented design.

UML class diagrams can show the important or relevant elements of classes, and the relationships between them, in a concise and intuitive way.

Other kinds of UML diagrams can illustrate how classes collaborate with one another and how users interact with class objects.

Most of our diagrams were created with a design tool called Umbrello.

For a good overview of UML, we recommend "The Umbrello UML Modeller Handbook," available from the help menu of Umbrello.