Use GET to pass Session ID

Hi guys,

I have seen some people use the GET method to pass the session ID generated instead of registering the username and other details once the user successfully logged in..

I am wondering does it make any difference ? I read a lot of reference books but none of them pass session ID like that and all of them do register the info the they need... however, i seen a lot of membership website do use GET method to pass the session ID instead of the registerd user name and etc... i guess yahoo mail also like that..

Please let me know what differences are..

Thanks