res.sendRedirect- this is the method which takes the absolute url and when this is executed it adds to the header the url and sends back to the browser client and it wud be treated as a new req then on i.e it makes a extra round of travelling back to the client when its executed.
RequestDispatcher.forward- this method takes the relative url and it just forwards the control to the specified relative url without again goin back to the client browser.
hope this helps if not im sorry
Cheers
Sri