Quote: "I've never understood this, in PHP, do you need to set a session ID?"
No. The session ID is generated automatically by the browser and the server. It tells the server which session file the user's data is located in.
I usually store simple stuff in there (that isn't worth querying the database for), such as "Logged In", "Is staff Member", "user ID".
Or if you were writing a shopping cart, you could store the items of the cart in that session.
Just call session_start() at the start of your php scripts, and everything is handled for you.
session_destroy() deletes the session data.
Sessions are automatically deleted after a certain period of inactivity (usually less than 30 minutes).