The following features are what I believe to be the key features of Charles. The best way to explore these features and to discover others is to download the free trial.
Charles can act as a man-in-the-middle for HTTP/SSL communication, enabling you to debug the content of your HTTPS sessions.
Charles simulates modem speeds by effectively throttling your bandwidth and introducing latency, so that you can experience an entire website as a modem user might (bandwidth simulator).
Charles is great for Adobe Flash developers as you can view the contents of LoadVariables, LoadMovie and XML loads. Charles also has native support for Flash Remoting (AMF0 and AMF3).
Autoconfigure Browser & System Proxy Settings
Charles will autoconfigure your browser's proxy settings on the following browsers:
- Windows system proxy settings (includes Internet Explorer and most other applications)
- Mac OS X system proxy settings (includes Safari and most other applications)
- Mozilla Firefox (on all platforms)