The definition of testing in software development is:
"Software testing can be stated as the process of validating and verifying that a computer program/application/product meets the requirements that guided its design and development, works as expected, can be implemented with the same characteristics, and satisfies the needs of stakeholders" (http://en.wikipedia.org/wiki/Software_testing).
Properly testing an application means that we will have to focus on every layer of the application being developed:
Unit/component testing: This verifies that a specific part or component of the application works as designed.
Integration testing: This verifies that the different components interact as expected. Also, this validates that interfaces between components are consistent to the expectations of other components.
Acceptance testing: This validates the final application behavior against stakeholder requirements.
System testing: This validates the application completely in integration...