Regression testing
The purpose of Regression Testing is to ensure that no new bugs have been introduced into the system after changes or fixes.
In fact, when a bug is fixed, Re-Testing is first conducted for the functionality that was fixed, and if everything is fine, Regression Testing starts.
Re-Testing
Verification that the bug has indeed been fixed – comprehensive tests for the functionality that was fixed.
Regression tests are usually performed after sanity tests (quick tests to ensure that the bug has been fixed).