conducting testing on the same build, is called retesting 
and 
conducting testing on the modified build is called 
regression testing.

as for my knowledge,

one performs testing on already tested functionality with 
different sets of values in order to ensure that the defect 
is reproducable or the functionality is working fine is 
called retesting.
one perform testing on already tested functionality in 2 
scenarios
1.whenever a defect is rised by test engr and rectified by 
the developer then it is released to the testing 
department,once the next build is released to the testing 
dept, the test engr will perform testing and complete 
already tested functionality in order to ensure that the 
old functionality remains same.
2.whenever the new changes are added to the existing module,
the test engr will perform testing on already tested 
functionality in order to ensure that its old functionality 
remains same.

Retesting: Re execution of our application to conduct same 
test with multiple test data is called Retesting.

Regression Testing: The re execution of our test on 
modified build to ensure bug fix work and occurrences of 
side effects is called regression testing.

a)	Retesting is nothing but same application with 
execution the multiple test data.
b)	It is so many phase doing in test life cycle
c)	Regression testing is nothing but re execution of 
the modified build is called the regression testing
d)	That when all bug are solved then time do this test
e)	Then that time developer or client had some changes 
that changes based test is called the regression test


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Retesting>>
    when a bug fix are Change Request is implemented during 
the Testing.Testing the Bug fix or Change Reguest is called 
Retesting.

Regression Testing
  In regression Testing we are Going to test dependences 
and Core functionalities.if it is working fine without 
effecting other functionalities (it means already Exesiting 
functionalities)then we can call it as Regression Testing...

If its not clear pls Mail me @obeldinesh@gamil.com.
           Thanqs in advance.....

Retesting--- Testing the application once again with the
initial test cases that is called Retesting

Regression Testing--- In order to check whether the
application has got any external impacts due to the new
fixes that is called regression testing

Retesting means the re-execution of the same build with 
taking multiple test data.It is conducted on the same 
build..

Regression testing means the re-execution of the selected 
test on modified build and co-related test to the selected 
test.It is conducted on modified build.

after the fix is provided by the developer he reassigns 
that to the tester.
first a tester test the fix provided by the developer is 
accordingly or not is called retesting
after the fix the tester has to do the retest and he has to 
find out the impacted areas is called regression
means the related testcases to the fixed one.
simply we have to execute all the major and critical test 
case of that functionality.