What is the the relationship between use cases and use case diagrams?

What will be an ideal response?

Use cases are a set of developed, structured use cases for the system or the subsystems that you are developing. It is likely that you have to revise the use cases later on, but this set must be good enough to sustain the next step of development: dynamic and structural modeling of the conceptual system. At least one diagram for the subsystem is necessary to summarize the uses cases and the relationships with the actors and each other. If a use case has too many "extend" and/or "include" relationships, you may create diagrams for the subset. But never, never feel that it is a good idea to create one diagram per use case. A use case diagram does not show what goes inside the use case.