sessions?

Just trying to understand sessions. For example, I have a user login page and a logout page. If 10 people are logged in and three of them log out - how does session_destroy() know which sessions to kill. I understand there are session id's - but not sure how they are assigned to each user and how they are controlled. Now what if I wanted to use another session for a chatroom lets say, same question as above - how are these controlled. I'm just trying to further understand how exactly sessions work so that when I am trying to use them I can do so more efficiently. If someone could explain this aspect of sessions, or point me to a good write-up, it would be appreciated. TIA