PHPBuilder.com has posted part three of their series looking at building a simple site using PHP and Ajax - this time with a focus on using the POST method instead of GET to push the data back and forth from client to server.
The advantage of this is primarily security, however you often can send larger amounts of data with the POST method as well (but that is dependent on your server configuration rather it be Apache/PHP, Microsoft/IIS, or Java/J2EE, etc). The primary advantage of POST is again, the security of the communication, because often log files on the web server will log the AJAX GET request data because that data is part of a URL in the request. This does not occur with the AJAX POST request.
They start with a simple example of the POST request type, pulling data from a form (address/city/state/etc) and pushing it to the backend script. They follow this by picking it apart, explaining each of the sections of code for a better understanding.