The document discusses the building blocks of the Unified Modeling Language (UML). It describes the key elements as things (abstractions), relationships (ties between things), and diagrams (groups of related things). The main things are structural (classes, interfaces, etc.), behavioral (interactions, state machines), grouping (packages), and annotational (notes). Relationships include dependencies, associations, generalizations, and realizations. Common diagrams are class, object, use case, sequence, collaboration, statechart, activity, and component diagrams.