What is :-

'Regression testing ensures that quality of software does not disturbed because of the changes is done.'

Regression testing is used to ensures effect of Fixed bugs in the Application. Which means when a tester finds the bugs in the application module and Programers Fixed-It, at that time Regression testing come in act to retest the same module. Regression is a Quality control plan which ensure that newly modified code complies with Specified requirement.

Why to do :-

'When you fix a Bug, It can create a new bug."

Every Module in Project are Related with each other. A Change in one module of application can effect other module. So, when a new modified Code is complied with unmodified code it will create or not create any new issue. Whenever changes happen to software, we do regression testing to ensure that there will be no affect on the existing functionality of application.

Note: Because of continuous Change in the application it is very important to do the regression Testing.