Session variables

I'm not too clear on HTTP headers, cookies, and such. So here are questions related to that. Let's say I generate a random number that I want the user to enter in a form. When I generate the number, I store it in a session variable ($_SESSION). When the user submits the form, I check the number they enter with what I've stored in the session variable. Since this session variable survives across page loads (assuming session_start() is appropriately called), how is it stored and recalled? Is it automatically stored as a cookie on the user's system? Or is it stored on the server? And how does a server "get" a cookie? Is it a separate request made by the server to the client? If the value I've asked the user for is *not* stored as a cookie, then is it passed as part of the HTTP submission or what? Thanks for any enlightenment on this. Paul -- Paul M. Foster