Is the site registration cookie based and if not how does it worked. When you come back to a site that you have previously logged into most of the time you are still logged in but not always. How long does the cookie or session last and would it be adjustable?

Also, the login block has the option for 'When login is complete, return users to this page.' What is this used for? If a user logsin will they just see the login block again since they are staying on the same page or can you display something else when the user is logged in?