HTTP Monitor
An HTTP Monitor is an application that enables the developer to view all of the HTTP traffic between your computer and the Internet. This includes the request data (such as HTTP headers and form GET and POST data) and the response data (including the HTTP headers and body).
An HTTP Monitor is useful as it enables the developer to see the hidden communications between the web browser (or other application using HTTP) and the Internet.
Charles is an HTTP Monitor implemented as an HTTP Proxy.