Regression Testing is to
test the unchanged part of the application which may be affected due to changed part.
We do regression testing after Re-testing of an issue. Regression Test cases can be selected by analysing the Impact Areas of the application which may be affected due to any other part.
Example Module A and B are dependent. Module A and C are dependent.
Bug resolved for Module B. then we need to test Module B for reported Bug. Also we need to test Module A and C because they may b affected due to changes in B