After taking the build from development team & completion 
of writing test case tester starts test execution, during 
test execution, if tester got any mismatches they will send 
that mismathes to development. then developers give the 
modified build to the testers. then they will do regression 
testing for completeness & correction.

Regression Testing: This is phase of testing where once you 
get a new build you would test the already existing 
functionlaity on the new build in order to make sure that 
nothing is broken
For Example: In Phase1(Build1) you have some functionality 
X.
In Phase 2(Build2) with the new build you would still want 
functionality X to be working.. so you text X win build 2--
>This is regression testing.

hi shailaja, after recieving that new build we test only
testcase that in which test case we got the error? or we
test that whole build?

hi deepa
     if we test the modified build with only failed testcase
 that is called Retesting
       if we execute all the test cases on the modified 
build inorder to ensure wheather any side effects effected 
or not is called regression testing

Software is modified for any reason, testing need to be done
to ensure that it works as specified and that it has not
negatively impacted any functionality --- Regression testing.