What is Unit Testing ? Level of Testing :
1 . Unit Testing : It also known as components, module or program testing. It is done by developers before the setup is handed over to the testing team to formally execute the test cases. It is performed by the developers on the individual units of source code assigned areas. The developers use test data that is different from the test data of the quality assurance team. Most often stubs and drivers are used to replace the missing software and simulate the interface between the software components. 
This Testing done before Integration testing
Limitations of Unit Testing Using this we cannot find each and every bug in an application. It is impossible to evaluate every execution path in every application. There is a limit to the number of scenarios and test data that a developer can use to verify a source code.
Test Basis :
Component Requirements
Detailed Design
Code
Test Objective :
Component
Programs
Data conversions
Database Module