System testing is the process of testing the AUT to 
determine whether the application is satisfying the user 
requirements.
it includes the following activities
Usability testing
Functionality testing
Load testing
Stress testing
Security testing etc.,

The input or source to System testing is The SRS.

Pease correct me if i am wrong
my id  rsntallapudi123@yahoo.com

I think ur answer is not complete.
The inputs to System Testing are
(1) SRS(System Requirements Specification) to prepare       
testcases and 
(2) developed software build (on which unit and integration 
testing has been done).

Integration Testing and Approved SRS or Functional Design 
Documents are the inputs for System Testing

Yes, Harikrishna given right answer. SRS, FD, USE CASES AND 
DEVELOPED BUILD

The inputs for system testing are

1. Integration testplan
2. Integration test case templete (entry and exit criteria 
followed for each stage of testing)
3.Requirement documents
4.FRS(functional requirement documents)

Muralidharan

dharanmu@yahoo.com

The inputs for system testing are 
1. SRAS, 2.FS, 3.The developed Build(Exe File).

Based on SRAS and FS only they will write the Test cases. 
The FS contains Use case Diagrams, Test statergy and test 
scope.

Please correct if i am wrong 

Pavan Kumar 
STC