Basant
There is lot of material on net among which these are very good posts
- Google page for Difference between user exit and customer exit This topic is more of a theoretical rather than a practical. I consider a Functional person need not get into it. In fact all the user-exits we talk about everyday, are theretically customer exits only (SE84).
- Enhancements and Modifications - BADI, Enhancement Framework, User Exits, BTE - ABAP Development - SCN Wiki
- Difference Between BADI and User Exits
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