Diagram   A diagram is a graphical device for representing a part of a UML model. A model is typically comprised of a number of diverse elements that are vertices connected by edges in a graph representing the semantic and structural elements of the model. A UML diagram will typically display a subset of the model and will commonly consist of a number of elements that have structural or behavioral significance when viewed together. A diagram has a frame that acts as a boundary to separate its contents-area from other diagrams and a heading which contains a string that lists the kind of diagram, its name and parameters of the namespace.

Explanation

When you are working with the UML you are typically creating new diagrams or viewing existing ones. Diagrams are the most common way of viewing a model. The model is the deep structure that underlies the diagrams and is typically stored in a repository. A diagram is a graphical representation of a part of the model and typically is drawn from a particular perspective and with a specified audience in mind. A diagram may be nested inside other diagrams and the frame in this case serves to separate the contents of the nested diagram from the elements in the enclosing diagram. Placing a diagram within another diagram is useful when the nested diagram has elements such as ports and gates that connect enclosed elements to the diagram frame. We see this in our daily lives when for example we are using maps. Often a large map will contain an insert of a smaller map to indicate where the area fits into a region, thus placing the more detailed map into an overall context.