use is something which the client gives or derive from FRS 

It is possible to write a testcase with out a use case ,but 
for that we need to have a FRS document

Used test cases are those cases which's related 
functionality is common in all the application. We 
use "Used test cases" to save our time otherwise writing 
the same test cases again and again is only wastage of time.
Example of Used test cases: login window, other contents 
could vary but the login window in all the application 
would be remain same.