Use cases are pictorical representation of your functional specification. This is a high level explanation and it is easy to understand.
Normally while preparing a test plan or test case, we use functional requirements and SRS. With the help of this functional requirements, we can derive USE case. In use case we will have actors and systems, where in these actors are used as external interface and the communicate to systems. Actors, can be end users, systems or even and input from a component, source etc. System is the one who gets an input from Actor to perform any function
Now days use case are becoming more popular for estimations
Use Case are nothing but user action versus system response. Use Case are usefull in combing the bussiness prespectives and implementation details and testing them together.
Use cases are design components, implemented during the design phase. Use cases represent the individual modules of an application and their interaction with each other. Each Use case diagram shows interaction between 2 or more modules for a given pre-condition.
Uses cases also show, how the application should react for a pre-condition with specific inputs.
