Structure diagrams emphasize what things must be in the system being modeled:
Behavior diagrams emphasize what must happen in the system being modeled:
Interaction diagrams, a subset of behavior diagrams, emphasize the flow of control and data among the things in the system being modeled:
The Protocol State Machine is a sub-variant of the State Machine. It may be used to model network communication protocols.
UML does not restrict UML element types to a certain diagram type. In general, every UML element may appear on almost all types of diagrams. This flexibility has been partially restricted in UML 2.0.
In keeping with the tradition of engineering drawings, a comment or note explaining usage, constraint, or intent is always allowed in a UML diagram.