What is a Use Case Diagram and what does it NOT describe?

It gives a quick high-level overview of which functions the system offers users — but NOT the internal implementation, the step sequence, or detailed responsibilities.

* A UML use-case diagram — actors (stick figures), use cases (ovals), and the system boundary (rectangle). — Slashme, GFDL / CC BY-SA, via Wikimedia Commons. *

A use case diagram lets you gain a quick overview of the functionalities of the specified system.

What it describes:

• Which functions are offered to the user by the system
• How these functions relate to other external interacting entities

What it does NOT describe:

• The responsibilities that the functions have in detail
• The internal implementation of the functions
• The sequence of steps within a use case

Use case diagrams show the "what" at a high level, not the "how" in detail.

UML notation basics:

• Actor: Stick figure (external entity)
• Use Case: Oval/ellipse
• System Boundary: Rectangle containing use cases
• Relationships: Lines connecting actors to use cases
  • <<include>>: Always includes another use case
  • <<extend>>: Optionally extends another use case

Go deeper:

• Use case diagram (Wikipedia) — the high-level actor/use-case view; shows what, not internal how.