Debasree,
'Work Site' is not a term in general use within SAP Production Planning software documentation. In my business experience, I have never heard this term used when discussing any PP topics.
So, I guess you could say that the "...difference between work center and work site ?..." is "anything you want it to be".
In the future, you should be aware that it is considered a poor practice in SCN to 'tack on' a post to a discussion that became inactive years ago. Some Moderators will even reject such posts.
Best Regards,
DB49