Thoroughly testing the whole application again is called regression testing.

So if 40 changes are made to the application we have to test the whole application again.

This is regression testing. In sanity we retest for one bug, we check the no unwanted changes are made related to that module. In regression the whole application is thoroughly tested again and again.

Regression testing is done whenever there is code change etc.If we fix a bug in module a ,we need to check all modules a ,b and c to see if bug fixed in module a has not caused unwanted changes in module b and c.

Regression tests should be best automated using a tool like Selenium webdriver.