$_POST holds post data, this is data held within the http header of when a page is requested.
$SESSION is a little more complex. A session ID is left with the browser (either in a cookie or a variable in the url) this session ID is used to identify which session file to load for this particular browser instance. The session file contains serialized variable data in the form
var_name|s:5:"words";
Where var_name is the variable name, s denotes that it is a string, 5 is how many characters is is and "words" is the actual contents of the variable.
On a *nix machine session files can usually be found in /tmp and the file name is of the form sess_[session_id]
HTH
Bubble
