A proxy is a piece of hardware that acts as an intermediary between the client and server. Application of software proxies as a way to debug web applications is a widely used practice, the main purpose of which involves inspecting data from requests and viewing the server responses.
On the Windows platform, we recommend Fiddler. It can be found at http://www.fiddler2.com/fiddler2/. If you are using a Mac, you can use Charles, which is found at http://www.charlesproxy.com/. Both are web proxies that record the communication between server and client.
Simply open the application, turn on the capture, and load your web application. Here you can see Charles capturing MovieNow requests:
It is possible to simulate responses using proxies as well. For example, in Charles you can go to Tools and then Map Local... to use local files or services as the response to your application. You can use Rewrite... in the same menu to modify part of a response and send it back to your application...