$_POST is only used to grab values that were submitted from a form whose method is post. As in:
<form action=foo.php method=post>
<input type=text name=bar>
...
</form>
When you submit the form, the values of your form elements will be passed to foo.php and can be retrieved from $POST[] variables. In this example, whatever the user typed into the text field named "bar" can be retrieved in foo.php with $POST['bar'];.
$_SESSION[] variables are set by you when you want a value to persist across several pages (as long as you call session_start() at the beginning of every page). Like so:
$str = 'some string';
session_register($str);
Now, for the duration of the session, any page that calls session_start() before any output to the page, will have access to that value in the form of $_SESSION['str'];
Hope that helps. Sounds like you need to look at some tutorials on sessions, and passing information between pages.
James
