Use cases defines the way in which end-user performs tasks in a application. Use cases are the first step for creating UML. based on the use cases the process UML is designed.

Use cases: mainly these will come into picture during design time.

First you will get the requirements and then you will think as end user of the product and then you come up with certain steps on those requirements. So, for ex. when you will be doing some medical related projects, you will think, how docter will use the end product, he will open project load some X-Ray image and do some mouse actions and unload it.

In software engineering a use case is a technique for capturing functional requirements of systems. Each use case focuses on describing how to achieve a goal or task. A use case defines the interactions between external actors and the system under consideration to accomplish a goal. Like a business use case describes the business process that is used by its business people. Use cases can serve as the basis for the estimating, scheduling, and validating effort. Means use cases put requirements in context, they are clearly described in relationship to business tasks.

Thanks
Gurjinder Singh Brar
http://dotnet-question-answer.blogspot.com