Use case diagrams are intended to provide all stakeholders, including clients and project managers as well as develops and engineers, with a high-level view of the subject system and communicate the highest level system requirements in non-technical terms.
The purpose of use case diagrams is to model what the system should do (What) without considering how it should be done at this stage (How) and to view the use of the system from the user's perspective (external view) rather than internally (implementation of these features).
Use Case diagrams have only 4 major elements:
The actors that the system you are describing interacts with:
The system itself (system boundary - the rectangle)
The use cases, or services, that the system knows how to perform, and
The lines (link) that represent relationships between these elements.
