After a test fails and a software fault is reported, the developer fixes the fault. After fixing the fault, tester again execute the test again to confirm that the fault has been fixed. This is known as re-testing.
Re-testing ensure that the test is executed in exactly the same way as it was the first time, using the same inputs, data and environment. If the test passes successfully, its mean the fault has fixed now. However we need to do some more tests. Because when developer fixed the fault, he might change some code, the changes may affect other code or other part of the software. So we test for unexpected side-effects. This is called regression testing.
