An important part of the Unified Modeling Language (UML) is the facilities for drawing use case diagrams. Use cases are used during the analysis phase of a project to identify and partition system functionality. They separate the system into actors and use cases.
Actors represent roles that can are played by users of the system. Those users can be humans, other computers, pieces of hardware, or even other software systems. The only criterion is that they must be external to the part of the system being partitioned into use cases. They must supply stimuli to that part of the system, and the must receive outputs from it.
Use cases describe the behavior of the system when one of these actors sends one particular stimulus. This behavior is described textually. It describes the nature of the stimulus that triggers the use case; the inputs from and outputs to other actors, and the behaviors that convert the inputs to the outputs. The text of the use case also usually describes everything that can go wrong during the course of the specified behavior, and what remedial action the system will take.
UML Use case diagrams are powerful tools for analysts to use when partitioning the functionality of a system. Use case relationships and the corresponding diagrams help analysts to structure use cases such that their textual descriptions contain a minimum of redundant information; thus making the whole text document much easier to maintain. But use cases are not design tools. They do not specify the structure of the eventual software, nor do they imply the existence of any classes or objects. They are purely functional descriptions written in a formalism that is completely separate from software design.
