Charles is an HTTP proxy, running directly on your computer.
Your browser or any other application with Internet access will be configured to access via Charles, and he, in turn, will show and display all received and sent data.

Developing under the web, you can not see what kind of data passes between the client and the server, and without this it is rather difficult to determine in what exactly the mistake was made. With Charles, you can easily see what is happening, quickly identify and fix the problem.
Charles allows you to debug your web applications quickly and reliably, saving your time and nerves!
Key features:

SSL Proxy - View SSL requests and plain text replies
Bandwidth Throttling for simulating slow connections, including delays
Debugging AJAX - viewing XML and JSON queries and responses in the form of a structured tree or just text
View as a tree of contents the AMF data that Flash and Flex are sent by attachments
Sending repeated requests to verify server response changes
Changing queries to verify the correct server response
Breakpoints for intercepting and modifying requests or responses
Checking recorded responses in HTML, CSS and RSS / Atom formats using W3C Validator