use case is a description of functinalities of an 
application in terms of actors, actions and responses .
once the test eng understand the use case he prepare the 
test cases.  use cases will be prepared by the business 
anlyst

in use case template we 
have following

use case name:  application name<eg:login screen>
functional requrements :all the functionalities<eg: 
usename,password, login, cancel, clear....in the lg in 
screen>
special requirements:how the functinalities will work when 
they invoked
use case template:it is document which has all above 
details in one.

let me show an example for welcome screen..

Actor Action                       System Response
1)Clicks on <New Applicant>      1)  System displays 
                                 Positions   Available 
2)Clicks on <Returning Applicant 2)System displays 
the                                Welcome back Screen 
 Agree & proceed> button
3)Clicks on <Decline & Exit>     3)System displays an 
 button on the                    Alert message with 
positions screen                  “OK” and “Cancel” buttons
4)Applicant clicks on ‘Cancel’   4)System closes the alert 
button                            popup and retains the
                                applicant on the same screen
5)Applicant clicks on ‘OK’      5)System closes the 
button on the alert popup        application and the browser


in this manner, but it is a tbular format..