The tutorial that I'm trying to figure out is this:
http://www.position-absolute.com/articles/jquery-form-validator-because-form-validation-is-a-mess/

The username field looks like this:

<input value="" class="validate[required,custom[noSpecialCaracters],length[0,20],ajax[ajaxUser]]" type="text" name="user" id="user" />

That <input> field has a class item named ajax[ajaxUser] which has its rules contained in this script (jquery.validationEngine-en.js).

The snippet for that item looks like this:

"ajaxUser":{
    "file":"validateUser.php",
    "extraData":"name=eric",
    "alertTextOk":"* This user is available",   
    "alertTextLoad":"* Loading, please wait",
    "alertText":"* This user is already taken"},

What I can't figure out at all is how its PHP page is working which is validateUser.php. Where are all those post fields coming from? I looked around and can't find a field with those names.

Thanks