simple session question PHP4

I'm totally new to sessions and I've been reading a few tutorials on how to use them etc. My question here that I have yet seen to be answered is this... Say you have two users come to a web site and you start your session with session_start() for one user and then you register your session variables with session_register() etc... and then say a minute later another user comes to your site and it calls the same functions; session_start() and then session_register(). My question is each session_start() call unique to each user that comes to the page, therefore you wouldn't lose any of the registered variables correct? Or do you have to do something different, like having a session id calling the session_id() function or a session name calling the session_name() function? The main thing I want to try and avoid is getting all the session variables mixed together with different users. Thanks, Jay Paulson