The document discusses Unified Modeling Language (UML) diagrams, which are used to visually represent systems through models. It describes the main categories of UML diagrams as structural, behavioral, and interaction diagrams. Specific diagram types are then defined, including class, component, deployment, object, composite structure, package, activity, state machine, use case, communication, sequence, and timing diagrams. Each diagram's purpose and how it visually represents different aspects of a system are explained.