Define Unit Test Case, Integration Test Case, System Test case

UNIT TEST CASES(UTC)
The unit test cases are very specific to a particular unit. The basic functionality of the unit is to be understood based on the requirements and the design documents. Generally, design document will provide a lot of information about the functionality of a unit. The design document has to be referred before a unit test case is written because it provides the actual functionality of how the system must behave, for given inputs.

INTEGRATION TEST CASES
Before designing the integration test cases the testers should go through the integration test plan. It will give complete idea of how to write integration test cases. The main aim of integration test cases is that it tests the multiple modules together. By executing these test cases the user can find out the errors in the interfaces between the modules.
The tester has to execute unit and integration test cases after coding.

SYSTEM TEST CASES
the system test cases meant to test the system as per the requirements; end to end. This is basically to make sure that the application works as per the software requirement specification. In system test cases, the testers are supposed to act as an end user. so, system test cases normally do concentrate on the functionality of the system, inputs are fed through the system and each and every check is performed using the system itself. Normally, the verifications are done by checking the database tables directly or running programs manually are not encouraged in the system test.
The system test must focus on functional groups, rather than identifying the program units. When it comes to system testing, it is assumed that the interfaces
between the modules are working fine.
Ideally the test cases are nothing but a union of the functionalities tested in the unit testing and the integration testing. Instead of testing the system inputs and outputs through database or external programs, everything is tested through the system itself. In system testing, the tester will mimic as an end user and hence checks the application through its output.
Sometimes, some of the integration and unit test cases are repeated in system testing also especially when the units are tested with test stubs before and not actually tested with other real modules, during system testing those cases will be performed again with real modules.