PHP AJAX POST or GET

xmlHttp.open("GET/POST",url,true)

[HeadBut]

I always thought a GET was for just GETting info and POST was for POSTing a new record to the Db/Site/Server. I've been trying to learn AJAX and it seems to use both the $_POST and $_GET vars no mater which I use: xmlHttp.open("GET",url,true) or xmlHttp.open("POST",url,true)
So, my question is, What is the difference and which should I use?

[coopster]

The two options are from the "method" attribute of the <form> element.
[w3.org...]