Page redirects or something else?

How does Facebook, for instance, "redirect" the user when logging into the site? For example, when you initially log in, you are on www.facebook.com, then the URL quickly changes to www.facebook.com/login.php, and then, assuming you submitted valid email/password, you are redirected back to www.facebook.com with your news feed, etc. Also to note, if your email/password are invalid, you are