what is basically a QUERY STRING ? how its beneficial and what is its scope ?

The query string is what gets passed at the end of the url. If the page url is "http://www.mywebsite.com/index.aspx?id=123" then there is a query string with a key of "id" and a value of "123". It's useful for telling a page to load specific information. It's scope is just for that specific request.

No,Query string manipulate through code,both key and value - as per above url example id of each user will be different and each time you pass differnet value to another page.

ok thanks