Definition: By knowing the inner logic the program the test what you conduct is Unit Testing. or Testing the piece/unit of a program

       Unit testing is a procedure used to validate that a particular http://en.wikipedia.org/wiki/Module_%28programming%29 - - source code is working properly. The procedure is to write test cases for all http://en.wikipedia.org/wiki/Subroutine - - methods so that whenever a change causes a http://en.wikipedia.org/wiki/Regression_testing -                    

Benefits: 

                        The goal of unit testing is to isolate each part of the program and show that the individual parts are correct. Unit testing provides a strict, written contract that the piece of code must satisfy. As a result, it affords several benefits.

            1) Facilitates Change

            2) Simplifies Integration

            3) Documentation

            4) Separation of Interface from Implementation

Limitations of Unit Testing:

                 Unit-testing will not catch every error in the program. By definition, it only tests the functionality of the units themselves. Therefore, it will not catch integration errors, http://en.wikipedia.org/wiki/Performance_testing - - software testing activities .

                 It is unrealistic to test all possible input combinations for any non-trivial piece of software. A unit test can only show the presence of errors; it cannot show the absence of errors. Though these two limitations apply to any form of software test.