Have you noticed that the example diagrams that we have shown so far in this chapter are all based on the same notation? It is called UML.
UML is a standard notation that defines a set of diagram types and visual elements that can be used to model systems and their different facets. Using UML, you can clearly model/document your systems and their structure/relationships. It can be used for both small and very large projects. The value of UML lies in standardization. Once you know it, you can more efficiently share your designs with other developers and analysts. Also, since it is a visual notation, it is also understood by business users.
In this book, we will only use the most common diagram type, which is called the class diagram (CD). There are many other types of diagrams that are definitely worth learning about, such as use case diagrams, sequence diagrams, state...