A use case is a description of the functionality (a specific usage of a system) that a system provides. The use case descriptions can exist in a textual form (a simple table), where the Use Case diagram provides additional information about the relationship between the use cases and the external users. The diagram also allows a definition of the system's boundary.
The Use Cases are described only in terms of how they appear when viewed externally by the user (a system's behavior as the user perceives it), and do not describe how the functionality is provided inside the system. The Use Cases are not object-oriented, but they are included in the UML to simplify the approach of the project's lifecycle – from the specification to the implementation.