Ashfaq,
I guess when you performed your search, (as required by the Rules of Engagement; see http://scn.sap.com/docs/DOC-19331) you must have somehow missed the extensive documentation contained in standard SAP help. This should always be your first source when trying to learn BASIC information about an SAP topic. Try
http://help.sap.com/saphelp_scm700_ehp02/helpdata/EN/48/03f68e67b34aa7e10000000a421937/frameset.htm
for a start. Then, if you still have questions, please post them.
Best Regards,
DB49