Where to initialize module specific variables I have been using hook_init in order to initialize some _SESSION variables on my application, however a problem I got after... is that hook_init is called on every ajax request. That's why I wanted to ask, what other hook, function or method should I use in order to initialize my module specific variables. I'm looking for something that's called just the first time the user enters the page, and not when a submit button is launched. (similar to isPostback???).
hook_init() is always invoked for not cached pages, even for AJAX request. You can verify the page request is made because an AJAX request, though. 

There are two conditions that should be common on any AJAX request:


The menu path is system/ajax
The delivery callback is ajax_deliver()


While the first condition is only true when the form element doesn't set #ajax['path'],  the second condition should always be true.

Code similar to the following one should detect when a menu callback is called because an AJAX request.

if ($router_item = menu_get_item()) {
  if (isset($route_item['delivery_callback']) &amp;&amp; $route_item['delivery_callback'] == 'ajax_deliver') {
    // It is an AJAX call.
  }
}


In Drupal 6, the menu callback for AJAX/AHAH requests changes, and each menu has its own callback. You can notice the path for those menu callbacks follows a specific pattern: It usually ends with /js, but that is not necessarily true. This means a menu callback could be associated with a path ending with /js, but that menu callback is not for a AJAX/AHAH request.