The RequestDispatcher#forward() forwards the current request to another page, where you can reuse the request parameters.
The ServletResponse#sendRedirect() redirects the response to another page, which is in fact just a new request on that page.
I would just add that you may try to work on your research capabilities. This is really asked countless times.
Here is one subtle example: http://www.google.com/search?q=What+is+the+difference+between+requestDispatcher.forward+and+response.sendRedirect