regression testing is that a devbeloper add certain 
features to the exsistant build then regression plays a 
vital role.
         say that u have version of 1.0 and u are going to 
do some enhancements for that, that means that  u r going 
change the version from 1.0 to 1.1 by cerain additional 
features

Thought not from a Project a generalized Example is Suppose 
u r testing a banking system. Firstly the system was build 
to handle checking accounts and developed and tested , then 
in a 2nd phase they developed Savings features and added on 
to the existing system , now if this has to be tested then 
we have to test the already exisitng cheking account 
functionality too. So testing all the existing 
functionality so that the new features have not disturbed 
the laready developed features is regression testing.