Use Case diagrams are used to identify the primary elements and processes that form the system.
Use case diagrams are great storyboard tools for user meetings. They define the requirements of the system being modeled and hence are used to write test scripts for the modeled system.
Visit the UML Use Case Objects page to get an understanding of all the objects used for UML Use Case diagramming.
A catalogue of the primary elements used fro Use Case diagrams that can be found in the Creately library are discussed in detail here.