user interaction with the application or system.it helps to
understand usability of the application.

It is nothing but user action and system response.  It
contains the flows typical flow, alternate flow and
exceptional flow. Apart from these it also has a pre
condition and post condition.

Use Case is simple way to express the funtional expression. 
Use Case are prepared by the by the business analyst. Based 
on the use case the test case are written.