hi ,
here requirements are give to the client .based on we write
the scenarios 
ex: login page 
scenarios on login page :
1.enter the url and click whether the page is open or not
2. the default focus cursor should be on user name
3. password should be encrypted format
4. password should not copied
5. input domain testing on user name and password
6. properties check on buttons 
7.functionality check of the buttons in login page 
8.authentication testing
9. check the session time out
10.check the cookies
like .....................

Take another example of date field.
1. enter the month > 12
2.enter the date > 31, 30,28 or 29 depends on the month end 
date.
3. enter year > or < the client requirement
4. Change the format and check
5. check dd/mm/yy with 0 values
7. check with negative values
8. if u have calendar object next to it, check it shows the 
same date when u r clicking.
9. check are u able to navigate in calendar object, month-
wise, year-wise.
and more.......

TITLE OF THE TESTCASE ARE CALLED TEST SCENARIO AND DETAILS 
OF THE TEST SCENARIO ARE CALLED TESTCASE.
example:Scenario
verifying that a user is able to manage his e-mail inbox 
such as view all messages, send a message, manage 
folders, logout…etc with a valid user login and password

test case:
is e-mail login page is displaying
is the user able to login using his login id and password
is he able to view his inbox
is he able to delete unwanted -mails…etc