The QueryString collection is used to retrieve the variable values in the HTTP query string.
The HTTP query string is specified by the values following the question mark (?), like this:
Link with a query string
The line above generates a variable named txt with the value "this is a query string test".
Query strings are also generated by form submission, or by a user typing a query into the address bar of the browser.
Note: If you want to send large amounts of data (beyond 100 kb) the Request.QueryString cannot be used.