Software testing is a process of making access to the functionally of a software program.Software testing is performed to verify that the completed software product functions according to the requirements defined by Clint.Software testing is the process of evaluation a software item to identify differences between given input and expected output.Testing is a process that should be done during the development process of software.Testing is the process of evaluating a system and its components with to find that whether it satisfies the specified requirements or not.Testing is executing a system in order to identify any errors , missing requirements or gaps in contrary to the actual requirements.Testing helps to improve the quality, reliability & performance of the system & also check that Software is not doing what he not supposed to do.It is better to introduce testing in the early stage of SDLC phases so testing helps the tester to identify the all types defects in the early stage & try to avoid the bugs.