Basant
There is lot of material on net among which these are very good posts
- Google page for Difference between userexit and customer eixt
- Enhancements (User Exits and Customer exits) - ABAP Development - SCN Wiki
- Difference Between BADI and User Exits
- Enhancements and Modifications - BADI, Enhancement Framework, User Exits, BTE - ABAP Development - SCN Wiki
You can go through several discussions on this topic, by searching through Google. I do not think there is need of repeating the same which was done several times in the past. In case of any residual clarifications, please revert.
Good luck
KJogeswaraRao