It is to test the application whether it has unauthorized 
access and having the enocoded security code.

eg:Considering banking application,it should have proper 
user login, there is no data extraction at the back end 
while using application.and also the application displaying 
on the site is also should be secured in case of online 
facility...i.e with https:

to determine the security of the software product.

The use of security testing is to make sure that our
software is secure enough .
Actually what i have done security testing so far i had put
different types of viruses in my application to check how
secure my software is .

Thanks
Akshay
 mail me @ : akshay_bali2001@yahoo.co.in