Unit Testing

Unit testing is the testing of an individual unit or group of related units. It falls under the class of white box testing. Unit testing is performed by the respective developers before the setup is handed over to the testing team to formally execute the test cases on the individual units of source code assigned areas. The developers use test data that is separate from the test data of the quality assurance team.
The goal of unit testing is to isolate each part of the program and show that individual parts are correct in terms of requirements and functionality.

Limitations of Unit Testing
Testing cannot catch each and every bug in an application. It is impossible to evaluate every execution path in every software application. The same is the case with unit testing. There is a limit to the number of scenarios and test data that the developer can use to verify the source code. So after he has exhausted all options there is no choice but to stop unit testing and merge the code segment with other units.