A use case diagram is a visual representation that illustrates how users interact with a system. It shows the different types of users, known as actors, and the specific tasks or functions they can perform, called use cases. This diagram helps to clarify the requirements of a system by highlighting what users need and how they will achieve their goals. In a use case diagram, actors are typically represented as stick figures, while use cases are shown as ovals. Lines connect the actors to the use cases they interact with, making it easy to see the relationships. This tool is valuable for developers and stakeholders to ensure everyone understands the system's functionality.