Testing levels are basically to identify missing areas and prevent overlap and repetition between the development life cycle phases. In software development life cycle models there are defined phases like requirement gathering and analysis, design, coding or implementation, testing and deployment.  Each phase goes through the testing. Hence there are various levels of testing. The various levels of testing are:

1. Unit testing: It is basically done by the developers to make sure that their code is working fine and meet the user specifications. They test their piece of code which they have written like classes, functions, interfaces and procedures.
2. Component testing: It is also called as module testing. The basic difference between the unit testing and component testing is in unit testing the developers test their piece of code but in component testing the whole component is tested. For example, in a student record application there are two modules one which will save the records of the students and other module is to upload the results of the students. Both the modules are developed separately and when they are tested one by one then we call this as a component or module testing.
3. Integration testing: Integration testing is done when two modules are integrated, in order to test the behavior and functionality of both the modules after integration. Below are few types of integration testing:
   • Big bang integration testing
   • Top down
   • Bottom up
   • Functional incremental (explained later under Integration Testing)

   Sometimes there can be several levels of integration testing :

   • Component integration testing: In the example above when both the modules or components are integrated then the testing done is called as Component integration testing. This testing is basically done to ensure that the code should not break after integrating the two modules.
   • System integration testing: System integration testing (SIT) is a testing where testers basically test that in the same environment all the related systems should maintain data integrity and can operate in coordination with other systems.
4. System testing: In system testing the testers basically test the compatibility of the application with the system. System integration testing may be performed after system testing or in parallel with system testing.
5. Acceptance testing: Acceptance testing are basically done to ensure that the requirements of the specification are met.
   1. Alpha testing: Alpha testing is done at the developers site. It is done at the end of the development process
   2. Beta testing: Beta testing is done at the customers site. It is done just before the launch of the product.

Other popular articles:

• What is Component testing?
• What is Integration testing? Examples, How To Do, Types/Approaches, Differences
• What is Big Bang integration testing?
• What is Unit testing?
• What is Coverage measurement tools in software testing?