Hi Kapil,
Refer http://scn.sap.com/thread/3217374
http://scn.sap.com/docs/DOC-27478
I suppose you are trying to learn the concepts now. The kind of questions that you post has either already been answered/discussed or documentations is available. So I would suggest you to search before you post, that way you are saving your time and get more articles from SCN for your reference. We do encourage to search for the answers first and then post your queries if you could not already find an answer.
Please read Rules of engagement
I would appreciate and encourage you to post your queries in SCN if you could not find it answered.
Thanks for your understanding,
Best Regards,
Leon