什么在jQuery AJAX中响应？

I'm sorry to add this broad question here, but I cannot seem to google it myself, I tried, and I really can't grasp the idea behind this, so here goes.
So I have a web-site that uses AJAX to login a user without full page reload. I kind of understand how AJAX works, and I'm sorry for my ignorance, but in my case:

$.ajax({
            url: './',
            type: 'POST',
            data: {auth: auth, login: login, pass: pass}
            success: function(res){
               // Code that checks if **res** is a specific string
            },
            error: function(){
                alert("Error!");
            }

I understand that this sends POST request to the same page with 3 parameters. What I don't understand is what specifically does it get as a responce? In my case, in res is the $_SESSION element that contains the string message.
My question is: how do I know what gets in the responce? If I would just echo something in my function, would that get in the responce? Is there like a documentation about what can be passed to the arguments of success function?
I'm really confused about this.

The "res"... or commonly "data" in most examples, is simply the reply data from your page that your posting to..

So say in the case of PHP... you yes would simply echo anything back to it.

commonly people use JSON, so with php you would create a array with all the data you want to send back and then simply do

YOUR PAGE THAT SENDS THE POST

<script>
    // JUST USING SUCCESS HERE ATM (Tthis does not show the full ajax command)
    // Refer to original question for full javascript
    success: function(res){
      var myData = $.parseJSON(res);
      if(myData.hasOwnProperty('name')){
          alert(myData.name);
      }
      if(myData.hasOwnProperty('object1') && myData.object1.hasOwnProperty('items')){
          alert(myData.object1.items.one);
      }
    },
</script>

YOUR PHP PAGE THAT RESPONDS

<?php
  $myResponse = array();
  $myResponse['name'] = "John Doe";
  $myResponse['number'] = 123456789;
  $myResponse['other'] = "and so on";
  $myResponse['object1'] = array();
  $myResponse['object2'] = array();

  $myResponse['object1']['name'] = "john";
  $myResponse['object1']['items'] = array();
  $myResponse['object1']['items']['one'] = "one one 1";
  $myResponse['object1']['items']['two'] = "two two 2";

  $myResponse['object2']['name'] = "jane";


  echo json_encode($myResponse);
?>

By using a "multidimensional" array in php, you can then treat each part of the array as a separate section/object

This might help: http://www.thecave.info/pass-a-php-array-to-javascript-as-json-using-ajax-and-json_encode/

Well, I think that what you "echo" is what you will retrieve in the "res", try to see it in the console with:

console.log(res); 

Or with an alert

alert(res);

try console.log(res); and check the browser console

ctrl + shift + k (firefox)

f12 (Chrome & IE)

For you task I would recommend using getJSON, instead of .ajax. It's just a shorthand for the same function, but really handy.

$.getJSON('/ajax-get-session/login/value/pass/value', function(json){
   if (!json.error) { //check if there wasn't error on the server side
       console.log(json.session);
   } else {
       console.log(json.error);
   }
});

And on the server side.

$response = array();
try {
  $response['session'] = $_SESSION;

}
catch (e) {
  $response['error'] = e;

}

echo json_encode($response)