Retesting:
First we test the total build. In that if we found bugs,we 
fixed and developers close those bugs.
Again we test the modified build. This second time testing 
is called retesting.

regression testing:
First we test the total build.After suppose any new 
functionalites are add to that build.We test these 
functionalites are effected on previous functionalites, is 
called regression testing.