In the UML, a diagram is the projection of a model from a specific viewpoint. It is the graphical presentation of a set of elements, most often rendered as a connected “graph” (drawing) of “vertices” (things) and “arcs” (relationships).
Hybrid diagram types are allowed – there is no strict separation among model elements. However, you are advised to use a selection of the standard diagram types unless you are expert in the UML.
Good diagrams make the system you are developing understandable and approachable. Choosing the right set of diagrams to model your system forces you to ask the right questions and helps illuminate the implications of your decisions.
The 13 diagrams found in The UML are primarily structural or behavioural diagrams. These are described in the next section.